Help with building a row of vintage lights
Other than the fact that having 4 in series means if one broke the rest can't turn on, it also means that each bulb will get less voltage. So assuming you have 4 bulbs that are identical and you put them in series, each bulb will only get 1/4 of the full power supply, thus if they're rated at full mains they won't turn on.

You could put them in series if you have bulbs rated at let's say 1/4 of the mains, then you will put 4 in series to divide the voltage by 4 for each one. Though make sure each one's resistance is identical enough, or the inaccuracies of the voltages may make some blow up.