The R4m is an upgrade for a previous build, and as such will only have partial instructions.
This is a shortened version of the R4 with a mag (hence the "m"). Shortening the barrel trades in power for handling so the R4m is much more balanced and controllable. It uses a snap-on magazine like my previous gun the RASP. All in all, it gets about 40' range with the ammo shown.
This instructable covers only the upgrade, it is strongly recommended that you view the instructable "R4 Stripestrike" for reference if you plan to build this.

Step 1: Changes to the Front

To start you'll need to remove two connectors in length from the top rail. Then you'll need to disassemble the front to the trigger and rebuild as shown.
I was lazy and only took pictures of the last 3 layers, but both sides are the same. So, good luck.

Step 2: Magazine

Snap-mag with an internal pusher. This one holds eight rounds of the orange ammo shown (same capacity as the original R4's turret).

Now you're finished with your upgrade! Congratulations, and have fun with your R4m "Reform"!