Representation is never not going to be lopsided. It's a WB crossover where they keep jumping around from CN, to DC to live action films, etc.
We can go on all day about Scooby and Daffy and however many other potential "should already be there" characters, but it's pretty clear that isn't how PFG is doing things.
Whatever the method to their madness is, it's just not something we're likely ever going to be privy to. Well, outside of the blatant promotional stuff, but even then Shazam and Beetlejuice are still notable characters regardless of their use as a means of advertising other projects.
Overall though, 4 characters really isn't that big of a deal imo, especially considering the circumstances surrounding BGs inclusion.
There might be a point to be made if we suddenly got data for IK and BP coming in seasons 5 and 6 back to back, but I highly doubt thats going to be the case.
I'm sure they'll slowly round out the Looney Toons and start adding other CN stuff. I mean they already are but characters are such a slow drip feed that it's going to take a looong while.