After digging old catalogues, here's the results
The 40k is the same one
Predator mk4 is the current version
the FW version is mkiVc if i recall correctly
according to Imperial Armour, there are 12 different marks known
After, i tried to dig the original box of the current predator, if i remember correctly the mark was mentioned on the box. it is possible that the current one is Mk3 and only the FW one is Mk4. As for the Epic version there's not so much a difference in design between the first edition and second edition.
As general note, when GW puts a mk number on a release they mix Epic and 40k releases together. it works for the Dreadnought (current plastic is mk5), the land raider (current plastic is mk3), the vindicator (current plastic is mk4) so far