Most people still playing MW4 Mercenaries are using MekTeks MekPak3.02b mod (110+ different Mech chassis and lot of new weapons). Microsoft has pulled support for ZoneMatch, so MekTek have made a replacement called MekMatch.
To install MP3.02b after a fresh install of Mercs;
1. Download and install Microsofts PR1 patch located
HERE. (about 5mb)
2. Download and install MekTeks PR1-MP3.02a patch located
HERE. (about 283mb)
3. Download and install MekTeks MP3.02a-3.02b patch located
HERE. (about 15mb)
4. Open your Mercs folder and run 'mekmatch.reg' to point the old ZoneMatch address to the new MekMatch address.
Installing the MekPaks does take time (on some slow computers maybe up to 20mins). During install, your computer may appear to be frozen. It isn't, it just takes a while to do it's thing. Just let it go.
If you have any troubles opening the game, copy the revised shellmt.dll file found
HERE into your Mercs folder.
There are still a couple of leagues running. just ask the guys you are playing with to let you know about them.
You can see who is on any server at any time using MekMatch's browser
HERE.