I rather see it as a solution to this thread's subject. In fact, we don't have a "fleet commonality problem", but a "fleet transitioning problem" (it's not about allowing us to fly 4 fleets, but to shorten the transitioning time requiring a 4th fleet).
I think we have both, and that they are two separate issues caused by the one rule.
One issue is fleet transitions.
One issue is the lack of flexibility and options in normal operations. Inability to run a smallish fleet because it seems like a fun or interesting challenge. Inability to react to what other airlines do. If you've got 50 f27-100s flying and 25 more on order as your 3rd fleet, and then realise the entire world is ordering f27-200s, your only real option is to join them. It's extremely impractical to look and see that the herald has just 4 orders, the HS 748 has just 20, or the Nord has literally no planes in existence, the ATL-90 has almost none, and both are dirt cheap and available right now. Because once you start flying that fleet, you're locked in. And you're locked in almost entirely because of the 4th fleet penalty.
From a personal standpoint, in the newest long GW, I won't fly a medium fleet longterm because of the 4th fleet penalty. But I could certainly use one for the next 10 years or so until larger planes start to be available. I'm locked into Viscounts because I chose them very early, got 38 of them currently. Production line is full, price of one is now 5 million. I could switch over to one of the cheaper options I've mentioned already, and I'm contemplating doing so. But it'd mean I'd need to save the cash to buy out the leases of the 16 viscounts I don't own and to order the ATLs, and then I'd need to wait the 2 years it takes for 38 ATL-90s to be delivered, leaving them idle on the tarmac the whole time, before changing over all at once. And because other airlines are locked in, I know that when I can no longer use a medium fleet, the 80 ATLs or however many I end up with will just be so much stored scrap, because nobody else will be flying them. Or I can just order more viscounts, fill the line for longer, increase the price a bit more.
It indirectly leads to a lot of very viable planes going entirely unflown, less variety, less interesting options. Admittedly that'd still happen to some extent even if it changed, and it's clear that many airlines don't think hard enough about their options, or don't take price & availability into account, which is why there are 50+ airlines into the f27 line, just 1 or 2 into the HS748 and Martin lines, and none into some of the other 40 seat props.
Which then leads to clogged production lines and the frustrations that entails, as we've seen some posts about.