No.4 bayonet is a socket bayonet, that fits over the muzzle onto lugs milled into the barrel. The No.5 bayonet is a more traditional bayonet, that mounts onto a lug under the barrel - that was a one piece mount that included the flash hider/front sight with a muzzle ring.
No.4 bayonets are anything but rare, generally. The exception being the No.4 Mk.I variant - which commands a much higher price. No.5 bayonets, especially wartime production are rarer than average - and command a price point around $350CAD. A proper wartime production scabbard will be marked S294 (Wilkinson) and will not have the large brass throat piece that is associated with the post war scabbards.
There are the cheap reproduction "RFI" marked No.5s around, they look like crap as far as I'm concerned but could act as a sort of stand-in... The Sterling SMG bayonets are a reasonable facsimile of the No.5, except for the ever so slightly smaller muzzle ring diameter, and the lack of a clearing hole - although I believe they're somewhat rare as well.