TBH you just have to get into the car and try the different seats you have out in different positions, and then you'll know what works and what doesn't, and where specifically you need more space (ie, what to look for in a replacement seat in terms of size or shape). It is often more to do with the shape of the seats and how they fit together than the actual size.
The Stages is pretty wide, so can be tricky to fit between other seats. It sometimes works better to get a slimmer belt fitted seat for the middle and have the adult on the side, rather than in the middle. If you do have your 4yo in the front, you want the seat as far back as it will go - TBH, I would be wary about having the airbag on because it is still likely to be more at his face level than his chest level, and the Stages is the older regulation so it has more tolerance for forward movement. If you do fit it in the front, make sure you really kneel into the seat when you tighten the belt to get it solid. I wonder whether it might work to put baby in the front RF in the Stages? Although appreciate this would not work if your DC tend to be smaller for age right from birth.
I recommended someone else the Britax Evolvafix recently as a FF seat, and this can be a good one for getting 3 across because it lifts DC up higher and the area around the seatbelt access point is fairly free - it worked well for us next to a belt fitted baby seat base in the middle seat (in a different car). You would need the Britax on one side, due to the need for the top tether. Although the harness limit is 105cm, you can use it slightly longer - my 4yo has just reached the point where he needs to switch to the seatbelt rather than the 5 point harness at 109cm tall.
Axkid Minikid seats are also RF and compact for fitting 3 across but they aren't the cheapest.
Britax King Pro is also a good forward facing belt fitted seat, although appreciate you might not want something which doesn't convert to high back booster. But if your 4yo is short as well as light, it might last you a while. I don't know how far up the headrest goes on it. It is on a Prime day deal so a reasonable price currently.