I agree with Keep Calm, in that a CR+4 encounter is, by design, 50% chance of a party wipe. If your parties can routinely beat CR+4 monsters, then they're either exceptional players or your DM is bad at tactics. In my playgroup, which was a mix of wargamers, who designed characters to win, and RPers, which were more haphazard and designed characters for coolness factor, we found that CR+2 was a "pull out all the stops and you can win" level of encounter. CR+4 was "pull out all the stops and and you might still all die." Even a CR+1 or CR+2 enconuter can kill a party member if you don't coordinate well, or if you have a couple bad breaks, so blithely saying that you should use CR+4 encounters normally bothers me.
One thing in particular that was addressed peripherally was the encounter composition of the CR+4 encounter. 1 monster of CR+4 vs a CR or CR+1 monster and the balance to CR+4 made up of low-CR trash is wildly different. For one, as Seraphim mentions, one or two fireballs will clear out most of the trash. So basically, the encounter hangs on whether the casters can launch AoEs before they get swarmed, and if they can, it's back down to a CR+1 encounter, which is fine.
On the issue of repetition: if you're delivering training, repetition emphasizes your point. This isn't training though. In this case, repetition makes it seem like you're getting paid by the word and just padding the column, or else you don't have a lot to say and want it to look more substantial than it is.