I assume you are specifically looking for compatibility with Wiha "System 4" style drivers? That's the system which is identical to the usual 1/4" (6.35mm) shanks, just smaller. They are a little uncommon outside of Wiha and random Chinese clones. The other big German manufacturers seem to prefer reversible style bits for their 4mm products. I tend to prefer sticking with the 1/4" stuff, even it is a bit larger, as the selection is much better and everything is interchangeable.
Felo series 280 or PB Swiss series 53 are their 4mm equivalents, but as I mentioned they are long-shaft, reversible types. Doubtful they'd work well with Wiha System 4 handles, but they are long and it might be worth switching systems if these bits are really important. If standard 1/4" stuff will do, look at PB Swiss "E6L" bits (95mm long typical).