Most of the characters in RE have little actual character development throughout the games... So I don't think it is necessary to play RE4 to understand him.. But you should because the game is good and will help you get into the series before the game.
Heck, I think I will play all the old games again except for RE5.
If you played RE2 and liked Leon then YES. He's changed quite a bit but generally for the better (considering he's been through a lot since Racoon City). --- I am yours to command, point out our foes and step out of my way. - Adam, Shining Force
RE4's one of those "love it or hate it" type of games.
If you're in to good game design, in depth plot, creepy atmosphere, lots of exploring, puzzle solving or zombies: SKIP RE4! You'll want to play REzero, Re1, Remake, RE2, RE3: Nemesis, or RE CODE: Veronica-X instead.
If you just want to run in a line for 5-9 hours and kill everything in sight: then by all means get it and enjoy. And you might enjoy RE5 as well (though a lot of people claim not to, even though it's actually more fun to play than RE4...)
On top of being a major let down in a series of majorly awesome games, RE4 also has almost nothing to do with anything even vaguely related to Resident Evil in general, except for Leon, Ada and Wesker. And Leon's characters is actually totally different from his RE2 persona. Granted, time has passed, and he's been through a lot, but there's litterally no sign of RE2 Leon in "mr cool" RE4 Leon.
Bottom line: You could go your entire life without playing RE4, even if you played all the other REs, and not realise that you missed anything at all.
It's existance is largely pointless. Both in that it's totally unrelated to it's own series, and also in that it's a medeocre action game at best.
So OP: I say skip it. Even if you're a newer gamer, you've probably already played better action games anyway. And Leon's character can be pretty well summed up if you watch Resident Evil Degeneration anyway.