Well, these days, at least at major junctions, daily trains and in most cases, non-daily trains as well, do not get a loco reversal, but get a loco change. This saves not just time, but also a lot of hassles for the controllers and makes their job easier.And it is quite common to have trains switching from P4 to P7 links at junctions like BZA.
And the loco change is completely dictated by the station where the change is happening, in this case, KGP. And due to it's proximity to SRC, there is always an abundance of P4 locos at KGP, which makes it logical to have them use the WAP4's for the rest of the journey...
more... when there is a reversal, instead of wasting time and man power on setting points, blocking lines and deploying crew to reverse the incoming loco and attach again on the other end. And in this particular case, with BBS Raj's schedule, it makes absolutely no difference if it gets a WAP4 or a WAP7.
And there is no 'official' loco for any train in IR, except Gatimaan and BPL Shatabdhi,both of which absolutely need a WAP5 for their schedules. Now, IR is definitely sticking to WAP7 for all it's Raj class trains, but this is mainly an exercise to provide consistent loco links for these trains to reflect their priority, rather than a requirement to maintain their schedules, which can be done well with a WAP4 as well. It is a requirement in the sense that, since HOG facility is being provided to WAP7 class first, and since Raj trains will probably be the first ones to switch HOG, it will be better to maintain their links consistently with P7 for now. If priority was the only factor, then NR would have given WAP5 to all the Raj's instead of downgrading their links to P7 and using the P5's for passengers in NR.