I went to J&G Sales 2 weeks ago and picked out a nice one out of three. Good bore, nice wood, no rust, gobs of cosmo, so I wasn't sure I liked it until I got home and cleaned it.
The other two that I looked at were rather dry and had the beginnings of rust/corrison showing on the metal. They would probably clean up OK. None of them had the "flip up micrometer sight", all had the "2 aperture flip up sight.
I'd say that if you don't already have a No.4 Mk 1, get it, although condition could vary