To be honest I think due to Covid, there hasn't been a need for a fourth DFDS decker since 6145 decided to go out in a blaze of glory. But quite possible some of these second hand vehicles coming maybe replacements for the DFDS Omnidekkas, those 4 ex EYMS B9TLs kind of match the numbers up nicely if they decided to go back to having for vehicles for DFDS (maybe the contract states that they have to have 4 buses painted).

At one point they had 10 buses dedicated to ferry work consisiting of.....
Leyland Atlantean / ECW - 3538/3540
Leyland Olympian / ECW - 3674 / 3744
Volvo Olympian / N-C Palatine 2 - 3832 / 3833
Dennis Trident / ELC Lolyne - 3866 / 3867
Dennis Trident / Plaxton President - 3890 / 3891
All except the Atlanteans were in the white and blue DFDS livery. The Atlanteans were in Coastline Livery with writing above the lower deck windows advertising that they were ferry shuttle buses.

It then dropped to 6 being 3832/3833, 3866/3867, 3890/3891.

Then the DFDS went for a refresh where it dropped to 4 branded vehicles being 3866/3867 and 3890/3891.

The number of branded vehicles did increase during the swap over between Tridents and the allocation of Omnidekkas 6144/6145/6147/6148. Though the Tridents weren't all replaced at the same time with 3866/3867 coming off first, and 3890/3891 a few months later.

Then it dropped to the current 3 with the demise of 6145.

As for GNE reusing fleetnumbers, it usually tends to be fleetnumbers in the 9xxx series that have been reused but I think they may have been on or two odd exceptions over the years, as per your example, though I don't think 3995/6 actually saw use with their originally allocated numbers. Mind plenty of vehicles have been renumbered over the years.