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G4 Bulbs

Every G4 capsule we hand-pick is a 12V lamp with two pins 4mm apart, the format used in under-cabinet lights, display fittings and low-voltage decorative lamps. Because they run through a transformer rather than off the mains, they stay cool enough for tight recesses and small glass shades.

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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.

Frequently Asked Questions

What is a G4 bulb?
A G4 is a very small capsule lamp with two pins set 4mm apart. It runs at 12V rather than mains voltage and is used in under-cabinet lighting, display fittings and compact decorative lamps.

Do G4 bulbs need a transformer?
Yes. G4 capsules are 12V lamps and need a transformer or LED driver. When replacing halogen with LED, an older wire-wound transformer may have a minimum load the LEDs cannot meet, which causes flicker or no light at all.

What is the difference between G4 and G9?
Pin spacing and voltage. G4 pins are 4mm apart and the lamp is low voltage; G9 pins are 9mm apart and the lamp runs on mains. They cannot be swapped for one another.

Why does my new G4 LED flicker?
Usually the transformer rather than the lamp. LED capsules draw a fraction of the power a halogen did, and many older transformers need a minimum load to run steadily. An electronic driver rated for low LED loads fixes it.

What is the difference between side-pin and back-pin G4?
It is about how the pins leave the body, which decides how the lamp sits in the fitting. In shallow under-cabinet housings a side-pin lamp lies flat where a back-pin one may stand proud, so check against your fitting.

How bright is a G4 LED capsule?
Modest by design, since these are accent and display lamps. Around 100 to 200 lumens covers most of what an older 10W or 20W halogen capsule produced.

How do I remove a G4 bulb without damaging it?
Let it cool, hold the body rather than the glass, and pull straight out. The pins are fine and bend easily, and a bent pin will not refit properly.

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