As a DM for 4th edition, I have a rather interesting bunch of players, but 4 of them that show up at my table tend to have extreme luck. 'W' and 'J' both have bad luck--there are nights where they cannot roll above a 6 on a d20, no matter how many times they try. On the other hand, 'K' and 'S' have extremely good luck. 'S', in particular, plays strikers with high damage, nearly always rolls above 15 for initiative, and has a habit of critting once or twice in a short combat. With his characters, (a monk with a vicious staff, or a slayer with a greataxe), he gets absurd amounts of damage on his crits, and often one-shots normal enemies this way.
What should I do to keep things fun and interesting for the table, when such extreme luck is going on? I don't want the high luck characters to be unchallenged, but anything that would threaten them, would destroy the low luck characters. Likewise, I don't want people upset at the high luck players--but even I get a bit frustrated by it. Help! ;)