6 and 4 key are for starting *Snow Leopard* in 64 bit mode - don't do that, Logic in 64 bits mode runs perfectly fine in SL 32 bits mode.
With SL in 64 bits mode, all your drivers and apps need to be 64 bit as well for them to work, and they (many) probably aren't (yet).
Summarize: boot SL in 32 bit mode, do the cmd-I on Logic and uncheck "open in 32 bit mode". Now you can run Logic in 64 bit mode.
Remember, SL 64 bit mode is
UNRELATED to Logic 64 bit mode. In fact, don't use SL in 64 bit mode at all, unless you know what you're doing and why.