I replaced 4 bulbs on a cooker hood with LED bulbs. Three will work ok till I put in the fourth; then they flicker once when turned on but will not stay on.
This usually means that the transformer either is not large enough or not compatible with your LED bulbs.
It could also be a problematic bulb or socket, so before replacing the transformer, try the bulbs in different sockets and see if its still always adding the 4th bulb in the same location that causes the issue. If its always the same bulb, regardless of number of bulbs installed or location, then that bulb is probably defective. Otherwise it's probably the transformer.
Note that some transformers/drivers, dimmers, etc. are rated differently for LED vs incandescent bulbs.
"it works with one incandescent" always means that the transformer or switch are more than just that. They are a semi-smart device which requires power for its own functioning: a lighted switch, motion sensor, dimmer, etc. etc.
They are built to use "the incandescent cheat" to power themselves. In this, they leak power through the incandescent bulb (which has nearly zero resistance when unlit).
When the lamps are all LED, this leakage power hits a brick wall in the LED's intelligent power supplies. It cannot leak, so it cannot work as designed.
The solution is to junk the cheating power supply or switch... and get a device which powers itself directly using its own access to the hot and neutral power, and then also provides a switched-hot wire to control the LEDs.