If you've graduated from one spool of PLA to a shelf full of PETG, TPU, and maybe a roll of nylon, you've probably hit the single-slot dryer bottleneck: only one spool stays dry while everything else slowly soaks up humidity. The SUNLU FilaDryer S4 is the popular answer — a four-spool box that dries (and stores) up to 4kg of filament at once. Here's an editorial, research-based look at what it does well, where it compromises, and who should actually buy it.
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This review is built from SUNLU's published specifications, the broad consensus across maker reviews and retailer ratings (the S4 carries a strong rating across roughly a thousand Amazon reviews as of mid-2026), and how it compares to smaller dryers. We did not run a multi-week lab test and we don't publish fabricated measurements.
What the SUNLU S4 is
The S4 is a dehydrator-style filament dryer sized for people who run multiple materials. The headline specs:
- Capacity: up to four 1kg spools (max spool size ~75 x 206mm)
- Heater: 350W PTC element
- Max temperature: 70°C
- Airflow: three internal circulation fans for even heat
- Humidity: built-in sensor with real-time interior humidity readout
- Feed: eight filament holes with PTFE tubes, so you can dry-while-printing multiple colors
- Modes: drying plus a power-saving storage mode
- Footprint: roughly 460 x 220 x 310mm, ~4kg
The practical pitch: load it once, keep four materials print-ready, and feed straight from the box so spools never re-absorb moisture between jobs.
Where it shines
Capacity is the whole point. If you're constantly rotating wet spools through a one-slot box, the S4 ends that juggling act. Four materials stay dry simultaneously, which matters most for the hygroscopic ones — PETG, TPU, and especially nylon.
Even heating. Three fans plus a 350W PTC heater is more thermal muscle than the single-spool boxes, and the airflow design is aimed squarely at the classic complaint that cheap dryers only dry the outer wrap of a spool.
Dry-while-printing for multiple colors. Eight feed ports with PTFE guides mean you can run a multi-color or multi-material job straight from the dryer. For anyone with an AMS-style setup who still wants dryness control, that's the killer feature.
It doubles as storage. The power-saving storage mode plus a sealed enclosure means the S4 isn't just a dryer you run occasionally — it's where your active materials live.
Where it compromises
70°C ceiling. 70°C covers PLA, PETG, ABS, ASA, and TPU comfortably, and reaches the low end for nylon. If your workflow is heavy on high-temp engineering materials (some nylons and PC blends like to be dried hotter), the S4 will dry them, just more slowly than a dedicated high-temp unit.
Size and price. It's a big box and it costs meaningfully more than a single-spool dryer. If you only ever run one spool of PLA at a time, you're paying for capacity you won't use.
Heat-up time. A large chamber takes longer to come up to temperature than a small one — expect to give it time before the interior is fully soaked in heat.
How it compares
| Dryer | Spools | Max temp | Approx. price | Best for |
|---|---|---|---|---|
| SUNLU FilaDryer S4 | 4 | 70°C | ~$130 | Multi-material makers, color walls |
| SUNLU FilaDryer S2 | 1 | 70°C | ~$45 | Single active spool, small desks |
| SUNLU S1 Plus | 1 | ~55°C | ~$36 | Budget entry, PLA-focused |
The decision is mostly about how many materials you keep in rotation. One spool at a time? The S2 or S1 Plus saves money and bench space. Three or four materials always on deck? The S4 is the upgrade that stops the juggling.
If you're still on the fence about whether any dryer earns its keep, our is a filament dryer worth it breakdown runs the actual math on wasted filament and failed prints.
Drying times and temperatures
A dryer is only useful if you use the right cycle, and "longer and hotter is always better" is a myth — too much heat can soften or even fuse some materials. As a general, conservative starting framework (always defer to your filament maker's guidance):
| Material | Typical dryer temp | Typical time |
|---|---|---|
| PLA | ~45-55°C | 4-6 hours |
| PETG | ~60-65°C | 4-6 hours |
| TPU | ~50-55°C | 4-6 hours |
| ABS / ASA | ~65-70°C | 4-6 hours |
| Nylon (PA) | ~70°C+ | 8-12 hours |
The S4's 70°C ceiling covers everything above comfortably except the hottest nylon recommendations, where it still works but takes longer. Note the pattern: PLA sits at the low end on purpose, because PLA softens early — running PLA at 70°C is how people accidentally weld a spool together.
Setup tips that matter
A few practical notes that turn a dryer from "owned" into "actually helping":
- Dry, then keep dry. Drying a spool once and then leaving it on an open shelf undoes the work in days. The S4's value is that spools can live in it and feed out during prints.
- Use the humidity readout. Run a cycle until the interior humidity stops dropping rather than guessing at a fixed time — that's the signal the spool has given up its moisture.
- Don't overfill the feed path. When dry-while-printing, route PTFE tubes cleanly so the filament isn't fighting friction on the way to the extruder.
- Give it room and a stable surface. A 350W heater puts out real warmth, so leave clearance around the vents and don't box it into a tight cabinet where heat has nowhere to go. A flat, stable shelf near your printer keeps the feed path short and the unit out of the way.
A common first-week mistake is treating the dryer as a one-and-done appliance. The materials that benefit most — PETG, TPU, and nylon — keep pulling moisture from the air the moment a cycle ends, so the real workflow is "dry, then store sealed or keep feeding from the box." Used that way, the S4 quietly removes a whole category of intermittent print defects that otherwise feel random.
Who should buy the S4
Buy it if you:
- Run three or more materials regularly (PLA + PETG + TPU is the common trio)
- Print hygroscopic filaments — PETG, TPU, nylon — that string and pop when wet
- Want to feed directly from a dry box during long or multi-color prints
- Are tired of rotating spools through a single slot
Skip it if you mostly print one spool of PLA and store the rest in sealed bags with desiccant. In that case a single-spool dryer is plenty.
Frequently asked questions
Can the SUNLU S4 dry nylon? Yes, at its 70°C max — that's adequate for many nylons, though some engineering nylons dry faster at higher temperatures. For PLA, PETG, ABS, ASA, and TPU the S4 has ample headroom.
Can I print directly from the S4? Yes. It has eight filament feed ports with PTFE tubes, so you can run multiple spools out of the box during a print, which keeps them dry mid-job. This is one of its main advantages over storing spools in bags.
Is a four-spool dryer overkill for a beginner? If you're printing one material, yes — start with a single-spool dryer. The S4 makes sense once you're juggling multiple hygroscopic materials and the single-slot bottleneck becomes a daily annoyance.
The bottom line
The SUNLU FilaDryer S4 isn't trying to be the hottest dryer on the market — it's trying to be the one that keeps your whole material rotation print-ready at once, and it does that well. For multi-material makers it removes a real daily friction. For single-spool PLA printers, it's more box than you need. Match the capacity to your shelf, dry your hygroscopic spools, and watch half your stringing complaints disappear.
The calibration sheet
New printers and materials run through the verdict rules the month they land — one email with what changed in the matrix.