I haven't had any trouble with wifi on the 4, with one caveat. The 400 (and maybe some of the later 4s) have an updated version of the radio. It just takes a new entry in ether4330.c and new blobs in /sys/lib/firmware. The entry I've got in my ether4330.c is:
{ 0x4345, 9, "brcmfmac43456-sdio.bin", "brcmfmac43456-sdio.txt", "brcmfmac43456-sdio.clm_blob" },
and the files named there are the ones I added to /sys/lib/firmware.
BLS