I use this http://www.pump.co.uk/generators-c108/petrol-silent-generators-c115/loncin-lc5000ddc-silent-petrol-generator-p5643For my 3600W vac you'll need a bigger one for the 4200W vac if you have one.Of course you are not allowed to use 240v if you want to pretend you are a professional. I do and have had no problems.
For our Gen1 a 5.5 KVA is needed, for our Gen2 a nice light 2.8 KVA will do!