B.W.
I would hold off on putting your 4yr old in a booster. If the Marathon isn't expired, keep using it until it is. I'm sure he has plenty of growing room in it as its a large seat that fits most kids to age 6. Until he outgrows it or it expires, don't move him to a booster seat.
For a convertible car seat (rear and forward facing) look for one with higher rear facing weight limits, and higher forward facing weight limits. the Graco MyRide 65 will rear face to 40lbs, and foward face to 65lbs. Its very comparable to the Britax Marathon size wise, and it fits babies and older kids alike very well. Other great seats are First Years TrueFit (rf to 35lbs, ff to 65lbs), Sunshine Kids Radian (rf to 40 or 45lbs depending on model, ff to 65 or 80lbs depending on model). Avoid the 3-in-1 seats like the Alpha Omega. The yare ok for rear facing, and ok for forward facing but are outgrown pretty quickly forward facing and they have consistently failed booster crash tests. There are better seats for the same or less money, so avoid them.
A rear facing car seat is outgrown when there is less than one inch of shell above the child shead, or the child goes over the rear facing weight limit. Legs touching the seat is NOT a problem. The harness should come from at or below the shoulders for rear facing. Google extended rear facing, Joels' journey for more information.
A forward facing seat is outgrown when one of three things happens, the child goes over the weight limit, the shoudlers go over the top harness slot, or the ears are level with the seat back. THe harness should come from at or above the shoulders for forward facing.
If your Marathon is expiring, consider the Graco Nautilus for your 4yr old. It will harness to 65lbs and then becomes a great booster.
For your 5yr old, absolutely a high back booster is safer than a no back booster. My 7.5yr old uses a Sunshine Kids Monterey, LOVE IT! Graco Turbobooster is a great booster seat, as is the Evenflo Big Kid Booster. Britax makes good booster seats too.