You sort of answered your own question.a 2-4hp engine is more powerful than a 1 hp engine.

But with the engines, the manufacturers do all sorts of arcane things to make their engine seem more powerful to the buyers. It is all marketing stuff.
The basic trick is to use a slightly more thin propeller which is a little too small for practical uses, and then use more nitro methane, and run the engine without the muffler so they can get the maximum RPMs that the motor can put out. Then they calculate its HP based on that. In real life with a realistic sized propeller and a muffler on the engine, it won't even come close to their advertised HP claim. Many of those so called 2 hp engines don't usually put out much more power than a regular engine does with all things being more equal.