The short answer: the NIIMBOT B4 prints 4×6" (100×150mm) shipping labels; the NIIMBOT B31 does not. If you are buying a label printer to handle parcels, that single fact decides it. The B31 tops out at 75mm wide, which covers almost everything except a standard courier label. Everything else about the two printers — 203dpi resolution, 80mm/s speed, inkless direct thermal, Bluetooth — is essentially identical.
The one difference that matters: label width
The B31 supports label widths from 20mm to 75mm and lengths from 8mm to 350mm. That is a genuinely wide range and it handles product tags, shelf labels, address labels, folder spines and barcodes without breaking a sweat. What it cannot do is a 4×6" courier label — the paper is simply wider than the mechanism.
The B4 supports 20mm to 110mm widths over the same 8–350mm length range, and ships with a 100×150mm starter roll precisely because shipping is its main job. If you print parcel labels for Shopee, Lazada, Qoo10 or your own store, the B4 is the only one of the two that will do it.
Where they are the same
Both print at 203dpi and up to 80mm/s. Both are direct thermal, so there is no ink, no toner and no ribbon to buy — the running cost is the label roll alone, like every printer in our direct thermal range. Both connect over Bluetooth to the NIIMBOT app on iPhone and Android and to Windows, and both handle barcodes, QR codes, batch printing and the app's template library.
Size, weight and battery
The B31 is the more portable of the two: 132 × 118 × 69mm, 451g, with a 1500mAh battery. The B4 is 157 × 135 × 69mm and 757g — noticeably heavier — but carries a larger 2000mAh battery to match. NIIMBOT designed the B4's roll bin to sit inside the body, so despite printing 4-inch labels it takes up around half the desk space of a conventional 4-inch printer.
The B4 also connects to up to 10 devices at once (3 iOS + 7 Android), which the B31 does not, and its printhead is rated to around 50km of printing. If several people share one printer at a packing bench, that matters.
Which should you buy?
Buy the B31 (S$96) if your widest label is under 75mm and you value portability. It is the natural step up from the 2-inch models for office and retail work — bakery ingredient labels, boutique product tags, jewellery tags and shelf edges. Pair it with the B31 label range.
Buy the B4 (S$120) if you ship parcels, full stop. It is also the better pick for a shared warehouse or fulfilment bench thanks to the multi-device support and bigger battery. Stock up from the B4 label range.
What about the K3?
If your printer never leaves a desk and you print in volume, look at the NIIMBOT K3 instead. It runs from mains power rather than a battery and prints at 180mm/s — more than twice the B31 and B4 — at up to 150 labels a minute, with a paper bin that takes both internal rolls and fanfold stacks. One important caveat: like the B31, the K3 maxes out around 80mm and cannot print 4×6" shipping labels either. It is a batch-throughput machine, not a parcel machine. See the K3 label range for compatible stock.
Quick summary
Need 4×6" courier labels? B4, and nothing else in the range will do. Need speed at a fixed desk without 4×6"? K3. Need up to 75mm and want to carry it around? B31. Still working out which size class you are in, our 2-inch model comparison covers the 20–50mm printers below these. And if your labels will live outdoors or get wiped with solvents, read do NIIMBOT labels fade over time before you commit to direct thermal.