Your DSLR probably is H.264, so you're already in the same ballpark as AVCHD. In other words, so long as your computer can handle four taxing streams of video simultaneously, you should have to transcode anything.
Have you tried just importing everything and building a multicam sequence?
When I put all four clips into a sequence (whether a 29.97 1080p DSLR sequence or an AVCHD sequence), the AVCHD footage looks extremely pixelated every 10 or so frames, to the point where faces are almost unrecognizable. However, the image below that was a great example of how pixelated faces got appears fine once I exported it using Adobe Media Encoder CS5 as a JPEG (the "bad" example looks fine below!". Not sure why that's happening either!
If the footage itself is "bad," then converting it isn't going to help. If, on the other hand, you're seeing a weird jerkiness/pulsing/break-up, that's likely because your playback resolution is set to 1/2 instead of Full. For AVCHD, this isn't such a bad idea, particularly if you're going to multicam--it'll likely let you edit and retain your sanity. However, to confirm that the footage is OK, set the Playback Resolution to Full and playback some footage or your sequence; things should smooth out from a picture stand point, but you'll likely start getting some stuttering playback. If playback stays smooth (temporally), go ahead and try to edit on Full.
These settings are for in-program preview only; they don't affect output one way or another.
That's exactly what it was. Thanks for helping out a first timer!