My information was outdated
I just spoke with CCI, and learned my previous information is out of date. Apparently the designs have changed since I last spoke to CCI about this (still can't recall how long ago, so I don't want to guess and give someone bad date/age information). The current information is that CCI 550 and 400 primers now both use different priming formulations and have different quantities of priming mix in them and also that the current 400 design uses a thicker cup than the current 550 design. So what I said before should be ignored and attempts to use the 400 in place of the 550 will require new load workups. I think CCI is exaggerating to say, as the tech did over the phone to me, that the substitution is playing with dynamite. I have never seen primers cause that much difference, but there may be some funny circumstances where this can cause a problem, so caution is important.
One thing I noticed looking at low-light primer ignition in the photos German Salazar made and published originally in his defunct Rifleman's Journal blog, as well as photos from a couple of other sources is that when
CCI changed their magnum rifle priming mix in 1989 for better igniting the high deterrent concentration exterior of the older spherical powder numbers, they included metal particles to throw white-hot sparks. Other primers at the time often did not do that, instead producing yellower or even blue flames without sparks. (KVB (Wolf, Tulammo) still did, last time I looked.) Subsequently, over the '90s and the first decade of this century, the standard rifle primers were changed to throw sparks, too, quite possibly because more spherical propellants became popular. Now you find folks reporting standard primers work fine with H110/296 much of the time, for example (anecdotal evidence, not a recommendation). So I am wondering if the change occurred when this switchover took place?
Anyway, my apologies for posting something out of date.
CCI informs me the BR-4 primers are nominally identical to 400s, and I have some of those that are less than 5 years old. I don't have any current 550s, but if I can get some, I'll set up and try my wax bullet test with both to see if I can get a clear indication of power difference.