>>716054797HOMM4 was an aborted mmorpg style abomination, but they ran out of money to finish it so they rushed up a HOMM3 sequel out of it. It has some interesting concepts but it plays a lot different from every other HOMM game. If that's the only game you played, you'll find the other ones strange since 1-3 followed exactly the same gameplay logic, each sequel just added more content, to the point where HOMM3 with its expansions had hundreds of everything. HOMM4 then totally changed everything. HOMM5 tried to go back to 3 and take only 1-2 good things from 4 and add one or two neat things of its own.
You should play 3 or 2. 3 with expansions is absolutely fucking stellar, 2 has the music and art style going for it but it feels a lot more constrained after 3.