Ken has been one of the most popular characters in the Street Fighter series since he was introduced.
That's because like Ryu, he can be fairly easy to get results with thanks to the familiarity of his moves.
The good news is that he hasn't changed too much in Street Fighter IV. He's not quite as good as Ryu but still has a few unique tricks up his sleeve to keep opponents on their toes...
Hadouken (quarter-circle towards + P)
The famous fireball. As with Ryu, use it to control the pace of the match and tempt your opponent into jumping at inopportune times. EX Fireballs beats Focus Attacks
Shoryuken (towards, down, down-towards + P)
Ken's Shoryuken has more range than Ryu's, which means he can intercept more jump attempts than his counterpart can. EX Shoryuken will see the move execute faster and reach even further, will connecting with a single LP Shoryuken while your opponent is in the air can be followed up by a HP Shoryuken before he hits the ground.
Tatsumaki Senpukyaku (quarter-circle back + K)
His hurricane kick is fairly useless by itself, it's best use is for EX Hurricane Kicks while in the air. Not only does this change the arc of Ken's jump, ruining any potential anti-airs, but it sets up a combo when it connects too. As with Ryu's, it is also good for escaping the corner but only the EX version is fast enough to get out of there without giving the opponent ample time to see the move and react to it.
Shoryureppa (quarter-circle forward, quarter-circle forward + P)
A series of shoryukens moving Ken forward, this tags onto the end of his combos with ease. Because his EX moves aren't that vital to his overall gameplan, it's often worth saving up for this super. You can throw a fireball beforehand and cancel that into the super (quarter-circle towards + P, quarter-circle towards+P) to increase the chip damage if you're looking to secure a win that way.
Shinryuken (quarter-circle forward, quarter-circle forward + PPP)
Ken shoots towards the sky in a upwards shoryuken. Needless to say, if someone is jumping so they'll land above you, it's a great time to use the Ultra. Otherwise, it's a little tricky to land - you need to learn some fairly advanced combos (good) or guess when to do it (bad) for it to connect.
Basic Combo: Jump in HK, HP xx MP Shoryuken
Advanced Combo: MP > HP xx LP Shoryken xx Focus Attack Dash Cancel > Shinryuken