Hi, I have 24 and 26" bmx cruisers, and Im getting the itch to build a 4x style bike. I get Decline magazine from the newstand now and then, wondering if there is a good web source about 4x?

Not really. Hardtail w/ 80-100mm of travel and short ChainStays, or a short travel dual-sus w/ around 4" all around, low BB, and a good balance on steering.
I am building up a new bike right now for Slalom.

I just found a link to huge 4x forum, loads of pics...
http://www.pinkbike.com/forum/listcomments...amp;pagenum=146

I'll get living hell from the ankle-biter set who read this, but PINKBIKE forums are not a true source of mainstays of set-ups and builds. Mostly 100lbs-when-wet teenagers who can hop onto anything and ride it. Just check out photos of current racers, spec out their set-up and frames, and find a medium.
If you are at or approaching 200lbs like me, you will find a dual-sus'er better when railing corners and slower sections.
Big tip is to mecanically lower the rear suspension to just above the recomended sag setting, and stiffen the heck outta it.

Well, dual-suss depends alot on your budget/local tracks. If you're tracks are all bombed out and filled with ruts, go the dually.

Spec wise, go for roady dearailleur/chain/cassette, something a little more substantial crank wise, like XTR if you have loads to blow. Chop your cassette down to 5 or 6 gears as well. Carbon bars (if you trust them), 6" rotors and even a 5" rear maybe. XC fork (Reba's are fantastic!!! so are the Fox line) etc etc.
Light as you can! Mine is about 12 flat now, with heavy-as cranks and heavy as brakes

Thanks for the info guys, Im trying to decide between a modified jump frame and a 4x specific. Those transitions look pretty nice, but all this stuff is adding up to some really big$ to build it up right.

A jump frame will do fine, just as long as it has semi-4X type specs (chainguide mount, vertical dropouts normally is the least you'd do). Make sure you spend TIME building it up. Wait for a good deal on a part to come through, and try to get the parts second-hand.
www.chainreactioncycles.com is fantastic aswell, incredibly cheap parts.