Prusa MK4S Complete Setup Guide: Assembly, Calibration, and First Print

The Prusa MK4S is Prusa's current flagship bed-slinger, sold either fully assembled or as a kit, and it replaces the MK3S+'s Bowden-adjacent E3D hot end with the in-house Nextruder direct-drive toolhead and adds a Loadcell-based probe and built-in accelerometer for input shaping. If you're coming from an MK3S+ or from a Klipper-based printer, several steps in Prusa's own first-run wizard are easy to rush past. This guide covers the kit-build essentials, the calibration wizard in the order it actually matters, and what to check before your first real print.

Assembly Notes If You Bought the Kit

The MK4S kit is a well-documented build via Prusa's interactive online assembly manual, but a few steps are worth flagging: the Nextruder's extruder gear tension screw should be snug, not maximally tight — over-tightening here is a common cause of ground filament and inconsistent extrusion on a machine that is otherwise dialed in. Second, dress the toolhead cable bundle so it doesn't snag on the X-axis idler at full travel; Prusa's cable channel is generous but easy to misroute on a first build. If you bought the pre-assembled unit, you can skip straight to first power-on, though it's still worth confirming the bed and gantry weren't racked during shipping by running the self-test.

First Power-On and the Calibration Wizard

The MK4S runs a first-time wizard that, in order, checks axis homing and belt tension, runs a selftest of the loadcell probe, performs XYZ calibration (squares the gantry to the bed using the probe rather than a manual paper-shim process), and finishes with first-layer calibration where you visually tune Z-offset via the live-adjust screen. Do not skip the input shaper / resonance calibration step even though it's optional in the menu — the MK4S has the accelerometer built into the toolhead specifically for this, and running it once at setup materially reduces ringing artifacts on fast prints compared to leaving default frequencies in place.

PrusaSlicer Setup

Select the exact printer profile for "Original Prusa MK4S" (not MK4, MK3S+, or a generic profile) so the Nextruder's direct-drive retraction settings and the loadcell-aware first layer compensation are applied correctly. If you're printing ASA or ABS, note the MK4S ships without an enclosure by default; Prusa sells one, and it changes chamber temperature enough that PrusaSlicer's stock ABS/ASA profiles assume it's present. Printing those materials open-frame without adjusting cooling and enclosure-dependent settings is a common source of warping complaints that aren't actually a machine fault.

Calibration StepWhy It Matters on MK4SHow Often Loadcell probe selftestConfirms first-layer squish measurement is trustworthySetup, then after any nozzle/toolhead swap XYZ / gantry calibrationSquares the gantry using the probe instead of manual shimsSetup, then if prints develop dimensional skew Input shaper (resonance)Removes ringing/ghosting at higher speedsSetup, then after moving the printer or changing part mass on the toolhead First-layer live adjustSets Z-offset visually per filament/nozzle comboEvery filament change if precision matters

Loading Filament and Your First Print

The Nextruder's filament sensor and load/unload wizard handle most of the mechanics, but confirm the extruder idler tension visually on the first load — you should see a light, even bite mark on the filament with no shaving. Slice Prusa's included calibration model or a small benchy-style test print rather than a full-bed job. Watch the first layer through the live-adjust Z screen; the loadcell probe means you should rarely need to touch a physical Z-offset screw, but the printer will still ask you to confirm visually since squish is a function of nozzle condition as well as probe data.

Troubleshooting Table

SymptomLikely CauseFix Loadcell probe fails selftestDebris on nozzle tip or toolhead cable snag affecting readingsClean nozzle, re-route cable, re-run selftest Under-extrusion on direct-driveExtruder idler over-tightened, grinding filamentBack off tension screw slightly, inspect filament for flat spots Ringing on fast printsInput shaper never run, or run with a loose gantryRe-check belt tension, re-run resonance calibration ABS/ASA warpingNo enclosure, profile assumes oneAdd enclosure or manually adjust cooling/chamber-dependent settings

Closing Thoughts

Most of the MK4S's calibration is automated compared to the MK3S+ generation, which removes a lot of the manual-shim, paper-feeler-gauge ritual longtime Prusa owners are used to — but that automation depends on the loadcell probe and accelerometer being trusted with clean, correctly-routed hardware. Run the full wizard once honestly at setup, keep the printer profile matched exactly to MK4S rather than a generic MK4 profile, and re-run input shaping any time you change the toolhead's mass or move the printer to a new table.

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