i assume when you say 4w that they are led? you need a driver for led lamps not a transformer.
A driver is only required for LED's which are not contained within a lamp and already controlled, drivers control current not voltage. However having said that there are voltage supplies which are called drivers in error.
Often LED units need DC rather than AC, I bought some cheap MR16 units for my bathroom, which have failed prematurely when powered from a transformer.
Many so called transformers are not simply a transformer although they do contain one. They use what is called a switch mode system to control their voltage. This switches (as the name implies) the power on and off and varies the ratio of off to on time (mark/space ratio) to control the voltage. This has a lower limit which is not zero as said often 20 - 100W.
My lights are powered from a simple transformer which is very heavy would make a good door stop but the switch mode high frequency versions are very light.
Because the switch mode high frequency versions turn AC to DC then back to high frequency AC then transform it to work with a dimmer it needs to talk to dimmer control so has to be special type.
The idea was that if a quartz halogen bulb is kept at exactly the right temperature then it will last longer and also be brighter and using 12 volt also means the filament is thicker again lasting longer so really one should not dim a quartz halogen bulb it defeats the whole idea of running them off a very controlled 12 volt output.
Once one stops using quartz halogen there is very little reason to use 12 volt. A LED GU10 spot light can typically run with anything from 140 volt to 240 volt there is no need for external voltage control.
It is claimed that some LED's will work with switched mode supplies
but this account shows in practice it does not work.
At one time we would simply say it will not work with a switch mode high frequency transformer but now there are versions which claim zero - 100W.
As to if a MR16 LED requires DC or AC or even what voltage it requires I have found nothing stating AC or DC neither have I found anything but the nominal voltage marked. i.e. 12v not 9 - 14 volt.
Since with the MR16 package there is no polarity marked in practice they must have a rectifier so they will work either way around. But AC at RMS 12 volt is more like 16 volt peak to peak. Since no tolerance is given we simply do not know if any damage will happen using a voltage which peaks to 16 volt. Likely not but only way to be sure is to supply with a DC voltage.
The correct name for a smoothed DC device is a power supply. However when my mother wanted a DC power supply for her door bell the electrician doing the job could only find one which plugged into a 13A socket. Well he called himself an electrician. I would guess if he had asked for a LED driver he would have got a DC 12 volt power supply it seems it common for them to be miss-labeled.
But of course you could also be supplied with a real driver which only limits current so one has to be very careful to read the specifications not just go by the name.
