I’ve bought a 350 mA converter, along with four Core 45° (7463685). Now I want to dim them – what’s the easiest solution?

If the converter you’ve bought isn’t dimmable, you’ll have to replace it. Choose a converter that is dimmable based on the load you have and how the lamps are powered.

In your case, you need a 350 mA converter that can handle 4 x 1.2 W = 4.8 W.