That’s a pretty good idea. Currently, the only BR range where players can achieve balanced offense and defense is BR 1–4. The leap to BR 5 is just too massive right now—no one dares risk being slaughtered by BR 5 vehicles while using BR 4’s less accurate, half-ammo semi-autos as primary weapons. At the moment, 1–4 is balanced, but BR 5 really represents a huge jump in quality. Often, when players finally reach BR 4, they get taught a brutal lesson by the overwhelming firepower of BR 5 veterans, beaten so badly they don’t even recognize themselves, and end up slinking back to the most balanced BR 3 to grind experience until they’ve fully researched BR 4.
Here’s the current situation: BR 1 can fight BR 2. Though the gap between them is big—like the appearance of APCs and large-caliber HE shells—such cases are relatively rare, and it mostly stays within the realm of bolt-action rifles going head-to-head.
Between BR 2 and 3, there’s a certain degree of parity, largely depending on player skill. Sure, there are issues—like Japan lacking Tier II aircraft that can spawn in the air instead of on carriers, and Germany lacking anti-tank options at BR 2 (missing the StuG III F/G, for example)—but overall, BR 2–3 is still playable. BR 3 only feels so good right now because hardly anyone plays BR 4. Think about it: compared to BR 5 weapons, BR 4 primaries not only have half the ammo but are also semi-automatic. That’s a bigger power gap than any previous jump in the game! Right now, the options are either introducing BR 6 and filling BR 5 with a bunch of new 10-round full-autos, while moving 15+ round full-autos, tanks like the Tiger II, M26, T-44/100, IS-3, and jet aircraft to BR 6 to create a smoother transition. But doing that would be exhausting and might require halting updates for half a year, so it’s unlikely to happen.
The situation with restricting BR 4 can be compared to Japan’s performance early after the merge: the U.S. had the Calliope, the Jumbo, the P-47… What did Japan have? Just one BR 4 copy of the Garand. Everything else was BR 2 anti-tank weapons and un-buffed BR 3 firearms. I’ve played it—it was like the Malevelon Creek war in Helldivers 2, with explosives, HE rounds, rockets, grenades, and full-auto fire never letting up, absolutely crushing Japan. Their win rate plummeted from a roughly 50/50 split with the U.S. before the merge to nearly 0%. Eventually, the developers had to step in and save Japan by locking matchmaking at BR 4, preventing BR 3 from being dragged into BR 4 matches. To this day, that protection—like an insurance policy—still shields Japan from things like the dreaded U.S. bombing runs.
Now, BR 4 isn’t as severe as Japan’s case was, but just check the developer’s backend data—BR 4 players are as rare as pandas. You either have fresh newcomers who just reached it, players gambling by sticking to BR 3, or people with a masochistic streak. I tried queuing with a German BR 4 squad to try and match against the USSR, but five matches in a row were against the U.S. We gave up using BR 5 vehicles like the Tiger II and the 42/22, and got absolutely suffocated by rockets and the T-20, to the point where we finally gave up on BR 4 altogether and just went back to either BR 5 or BR 3.
So, based on actual cases and my personal experience, I support putting a separation between BR 4 and BR 5. This would save BR 4, restore a normal experience for players grinding through the tech tree, and keep BR 5 fun enough that people will still play it.
The last time they actually promised they’d do something about it they gave us the “preferred map/mode” shit show, and so far have done NOTHING I know of to correct it. So, I don’t foresee them doing anything about it until the player numbers drop to the point they get desperate. 
