Why are LED bulbs in a cooker hood flashing when I put in the fourth bulb?
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.