Nick 323 - Convert by all means BUT be aware that the dip wire may be live all the time the headlamps are selected so when you select main beams you will have both elements working in the H4 envelope. Not good - a rewire will be required. Good luck mate.
Good - Fast - Cheap: Pick any two.
E23 735i; E32 735iL (X 2 - 1 Alpine White and 1 Glacier Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E30 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i