G4 Bulbs

What a G4 capsule bulb is

G4 names the cap: two pins set 4mm apart on a very small body. It is the smallest of the common bi-pin lamps, which is why it turns up in under-cabinet strips, display cabinets, plinth lights and compact decorative fittings where nothing bigger would fit. In the US the same lamp is described as a G4 bi-pin.

G4 is low voltage, so check the transformer

A G4 runs at 12V, not mains, and needs a transformer or LED driver upstream. This is the detail that catches people out when swapping halogen for LED: an old wire-wound transformer often has a minimum load far above what a couple of LED capsules draw, so the lamps flicker, buzz or refuse to strike. An electronic driver rated for low LED loads solves it. If a replacement lamp seems faulty straight out of the packet, the transformer is the first thing to check.

G4 or G9?

Measure the pin gap. Four millimetres is G4 and low voltage; nine millimetres is G9 and mains. The lamps look alike at a glance and are not interchangeable in either direction, so it is worth checking before ordering rather than after.

Brightness and colour

G4 capsules are modest by design — this is accent and display lighting rather than room lighting. Compare lumens rather than watts when replacing halogen: something near 100 to 200 lumens covers most of what an old 10W or 20W capsule did. Warm white around 2700K suits cabinet and display use, while cooler temperatures make the light feel more functional under a worktop.

Pin orientation and fitting

Some G4 lamps are side-pin and some are back-pin, and the difference decides whether the capsule sits flat in a shallow recess or stands proud of it. Check which way the pins leave the body against the fitting you have, particularly in under-cabinet housings where clearance is measured in millimetres.

Care and handling

Handle a G4 by its body, never the glass, and let it cool before removing. The pins are fine and bend under very little force; once bent they will not seat cleanly. Pull straight rather than rocking the lamp out, and keep anything damp away from the pins and holder.