Does anyone know who has a 4-position FCG set for RDIAS in stock?
Also any source for DPMS 4-Position Trigger Kit w/ red spacer?
Any help would be greatly appreciated.

If you have a form 4 or dealer call Remington for DPMS kit, when they were just DPMS they would sell the sets direct to customers. Maybe check brownells as they carry Dpms parts.

Wasn’t it M60 joe or quarterbore who did the mods? It’s been forever since I’ve seen it but haven’t looked also.

Specialized Armament usually has the needed parts.

SAW

+1 I got mine from them, however you can piece your parts together from other sources and save money.  Example SAW has the Trigger for burst/4-way at $50.00 yet you can get the same thing from Brownells for $35.

The only thing you need from SAW are the following 4 parts: 4-way Sear, 4-way selector, 4-way left disconnector, and 4-way right Disconnector.  All the rest of the parts are the same as the 3-way Burst parts, which you can order from Brownells.

Great info but mine is RDIAS, and I can’t get the 4 way sear.
Is it still going to work with my RDIAS with following 3 parts and obviously minus the 4 way sear?

From a few years ago:

FWIW the picture above is for the DPMS 4 way kit not the Colt 4-way.

Yes should work with your RDIAS.

Oh and something I forgot to mention, you won't have to worry about this with a RDIAS, but those of us with a Register receiver do.  For the RDIAS host receiver you will have to remove the selector stops.  For those with a registered receiver.  You need to cut the switch part off the selector and JB Weld a cut down cheap Selector switch on to it to be able to rotate it 360 degrees.

Edit to add:
There is a thread that is pinned/sticky in this section that pretty much covers all the questions about the 4-way
https://www.ar15.com/forums/Armory/M16-4-Way-Selector-Install/23-364516/

Well considering the selector stops aren't ground down there is no way it could even fire in burst mode

It's hard to tell for sure from the angle, but I think that photo may be of the DPMS version which has an extended length selector so that the stops don't have to be ground down.  Of course that gives the down side of the selector being more likely to inadvertently being knocked into an undesired position.

MHO, YMMV, etc.