Let's say some of the best lighter weight defensive fighters had to box against one of these huge NFL freak athletes. It wouldn't be sanctioned in the first place but hypothetically let's entertain the idea. The smaller boxer would largely benefit from a 4 rounder because it would surely increase his chance of going the distance. They would need to fully use their defensive skills, speed, reflexes, ringcraft, footwork and agility to evade their MUCH larger, stronger and more powerful opponent. One round of the larger man swinging and missing isn't hard to imagine, or even two or three, but eventually the smaller boxer will make a mistake and it's likely to only take one in such a scenario.
Relying on blocking isn't going to work when a guy that outweighs you by 100+ pounds is hitting your gloves with power shots, they'll blast through your guard, but evasive defense could work for a limited period of time or potentially even carry an elite defensive boxer the distance in a professional 4 rounder (those chances would be low imo). The larger man would be hellbent on cutting the ring off to walk the smaller boxer down while it would be the smaller man's job to avoid being cut off and to consistently escape with their consciousness. A much quicker more agile man with excellent footwork to prevent their ring cutting and their ability to find escape routes when momentarily in bad positions (against the ropes or stuck in the corner) would make the much larger man giving chase have to work hard to track them down.
The smaller man would have to hope that their opponent would gas out early or simply stop giving it full effort if they're not able to find any success & frustrate the shit out of them. A persistent much larger man would be far more likely to eventually catch them and due to the ridiculous size difference their increased puncher's chance would almost surely be a one-and-done affair. They would have to remain focused & persistent and make sure they don't punch themselves out knowing that the smaller man's margin for error is so thin. Ring size would be something that would need to be factored in as well.