Yes we have the right class to drive it so no worries there. My other half is in his favourite place behind the wheel of a left hand drive motorhome. We haven't had this one for long so he's currently doing a few things to it, to make life easier for me. We suspect, as you say that it might be 4200kg but we will post some pics over the weekend & maybe someone will be able to tell us from the plateI assume it's a 400i, not 4001, as I couldn't find the latter so I've had a shuftie at 400i models and they seem to be 4200kg so it could be that the one you have was originally at that weight and down-plated by a previous owner.
Van Weight Engineering as already mentioned can do the paperwork necessary to get it back up to 4200kg but make sure you have C1 on your driving licence to allow you to drive over 3500kg otherwise you'll be going no-where.
The converter's plate (ie Laika) is the one you need to look at.