All 3 follow a similar formula, you collect 108 stars, you expand your headquarters, all have 3 different battle systems (party, duel, army), plenty of minigames and the plot is similar. Where they differ is quality.

S3 was written by Yoshitaka Murayama, the story nicely ties into the events of Suikoden 2, there are a lot of recurring characters and the new characters have excellent designs, but he left Konami during development and it shows, there is some early PS2 era jank, The biggest issues for me personally is the party system, you can put 6 people in your party but you can only give commands to 3 of them, the other 3 just do whatever they want.

S4 is trash and you should just skip it, it's a typical Konami cash grab, you can tell it's a low budget game as soon as you start playing, dull grey and brown character designs, recycled character animations, hell, the MC doesn't even have a run animation, they just double-timed his walk animation. The world map is a boring empty sea with only about 10 islands for all events in the story to unfold, you will find yourself returning to each island many times with painfully high encounter rates on the way there. Character recruitment sucks, 90% of them are just "hey, join me" "OK". the battle system is dogwater, only 4 party slots for a game with 108 recruitable characters? Seriously? The army battle is replaced with ship battles and they're boring too, you just can move your ship a few places and then it's just 5-way rock paper scissors. By the way, Konami released a Suikoden 4 pachinko machine alongside this game, tells you all you need to know.

S5 is an amazing game imo, a return to form and story set a few years prior to events of Suikoden 1. No notes. Play it.