I've never been a big fan of Gr4 to be honest, I've always felt like they have too much grip for the power they have and suspect they would race a bit better if they were on a less grippy tyre, sports soft would suit them a bit better in my opinion (without testing it). However the cars need to be better balanced as they are for the people who do race them, it seems PD gave up on the class a few years ago when they dropped them from the eSports for the top division (aka the only one they're interested with).
The class should be broken up, it would be a lot easier to BoP if they were matching FWD cars against each other, etc (not just on raw pace but over a race with tyre wear) and would give more variety instead of using the same cars all the time.
Gr2 and Gr1 are all over the place with different types of cars, I don't think they should ever be used as categories for sport mode but ran with the specific cars allowed only, so in Gr1 they could set it to GrC only on certain weeks or 2008 GT500 cars on Gr2 weeks. The BoP in Gr3 isn't perfect but it's close enough for most cars to be competitive in the races (even when the leaderboard is showing the usual suspects dominating), the oddballs like the Skyline Silhouette don't belong in there.
I can't see PD doing BoP for certain cars each week (I'm not sure it's an option since the BoP changes so far always need a game updated) but if the classes were smaller it would kind of force their hand.