My derailleur model will accept both 5- and 4-speed freewheels, although it recommends a 1/8" wide chain for 4-speed and 3/32" for 5-speed. I have to put more miles on it to see whether the r.d. performs reliably with the 14-19-24-28 block, but so-far, so-good. If I have to fidget and fuss a bit, it is worth it to me on this project, to be able to keep the bike as "period" and as British as possible.
If I enounter insurmountable problems, I'll search in earnest for something period, but with a more up-to-date design, like the Simplex RaidExport 32 2nd version. That particular early-60's derailleur is rated for 32T.