I've just purchased what was advertised as a Yamaha Fazer 400 headlight unit as I'm aware they can be modified to fit the 600 (which I own) and have better lenses than the 600.
Is it the headlight unit you need help with or the bulb type..?
Sorry for the delay in responding to all the above. I realised the inactivity on this part of the forum after posting and didn't think anyone would respond.
Neither, now; to answer the above question.
I realised shortly after purchase that the unit that the seller had advertised was a Fazer 600 MK1 box-eye unit (so the same as the one I have currently fitted). I canceled the order and informed them (not expecting someone to know if all they've done is removed it from zed bike, or split it from a job-lot) and they decided to re-list with the exact same title despite it being for a FZS 600. Sure it will fit a Fazer 400 but there's a difference between the two units (as we know).
For now I'm running with both headlights wired in via a simple wire I got pre-made from eBay, but had read that the Fazer 400 O.E.M headlight unit (that I've since obtained pictures of) offered better reflectors and spread (but are super rare).
I'll see how far into winter I get before the headlights bother me.