Why not take them back, and get 4x 240V downlighters. They are dirt cheap,and you can get 240V downlighters rated for bathrooms and damp areas. IMO much cheaper and safer than trying to source a transformer.
I,m not an Electrician, but my bathroom hasn't set alight yet.
Failing that, any of the B&Q type places will sell them, you dont actually have to have one per light fitting, or indeed one for all, as long as the W = Transformer output, you are fine, but word of advice, always go a bit above the required, ie, if it totals 200W go for a 250W transformer and so on.
Same goes for dimmers, always go that bit above required usage.
Also, you can always change the 50W lamps for 30W.
On the note of 240V, it depends where they are located in your bathroom, ie which zone they are in.
If you do have one above your shower, then it needs to be IP rated to suit, and also LV, so stick with the LV and get the transformers to suit.
Any light fitted as an integral light/extract above a shower has to be IP rated to suit and LV, ie 12V, regardless of where the fan is, and usually in this situation, in the loft.
If yours is 240V then it fails regulation, and is therefore illegal, and will nullify your house insurance should it go up in smoke because of it.
Just as a word of caution, just because B&Q sell it, doesnt mean it is within the regs.
Sorry to sound all jobsworth, but the building regs are shite at the best of times, and i have to pay a lot of different agencies, to be able to my job, (if I were Polish it might be different) but those of us who do this for a living have to follow them.
And as such, you cant just fit anything, regardless what B&Q sell you.
Havin spent the past 8 years of my life sorting downlighters for auld wifeys id recomend 230 v 35 w lamps forget about the transformers as long as there outwith zone 3 in a bathroom transformers love to die on you as they wish i have six in my kitchen which havent been change for about three years there still running strong