Hmm, I've been marathoning B5 for a newbie and we finished season four a few weeks ago, then took a break. Next week I was going to show him 'In the Beginning' before we started watching season five.
It's been a while since I watched either ITB or S5, so why shouldn't I show him ITB before S5? Didn't it air right before S5?
It should be perfectly fine to watch between S4 and S5.
As for the recommendations to watch it between "Objects at Rest" and "Sleeping Light"... well, I guess that's because "In the Beginning" (or at least its framing story) takes place between these two episodes. And thematically (as well as emotionally) it fits very well with the series finale.