Flat 4 engines are not subject to the porpoising issue. The problem is when the 1st and last cylinders (inline 4 cyl) fire at certain RPMs the forces trying to push the cylinder head in the opposite direction of the piston become harmonized and it amplifies the resonance when in harmony, with a fore and aft rocking motion.
This is especially prevalent in FWD cars that do not have a transmission bolted to the engine that is another 3 feet longer with a good distance between the engine and tranny mounts.
It did not seem to be a problem in the old days and many early 4 cylinder engines were a heck of a lot larger displacement than 2.4 liters.
Offenhauser - Wikipedia, the free encyclopedia
Big 4 cylinder high performance engine. Copy of the 1913 Peugeot Indy engine that was DOHC 4 valves per cylinder hemi (I think it was a Hemi).
regards
Mech