A few notes:
1. If you connect your heated bed sections in parallel, they will take 14A each at 12V (perhaps a little less after voltage drop in the wiring). That's 56A total, close to the limit of your ATX PSU, and you haven't allowed for the current draw of stepper motors or fans yet. Unless your heated bed is huge, you should get enough power if you apply 12v to two sections connected in series, and the same for the other 2 sections. That brings the current down to 14A total and the bed power to 168W. That should be sufficient to heat a bed up to about 250mm square to about 120C.
2. Some ATX PSUs have a single 12V supply internally, others have 2 or more. The label on the PSU should say. If your PSU has 2 or more separate 12V supplies internally, you must not connect them in parallel.
3. If you really want to drive all 4 sections in parallel, then you would need a 56A switch to control them. Your RUMBA board won't handle anything like 56A, so you will need to use a 56A external switch. That in turn would require several mosfets wired in parallel. So instead, I suggest you use 4 external switches, each rated at 14A, all controlled from the same RUMBA output. Then you can connect each heater section to a separate output from your ATX PSU, solving tour wiring problem.
4. If you do run all 4 sections of your heated bed from 12V, that's 674W of power in total. That represents a serious fire risk, for example if the firmware freezes and leaves the heated bed at full power.
1. If you connect your heated bed sections in parallel, they will take 14A each at 12V (perhaps a little less after voltage drop in the wiring). That's 56A total, close to the limit of your ATX PSU, and you haven't allowed for the current draw of stepper motors or fans yet. Unless your heated bed is huge, you should get enough power if you apply 12v to two sections connected in series, and the same for the other 2 sections. That brings the current down to 14A total and the bed power to 168W. That should be sufficient to heat a bed up to about 250mm square to about 120C.
2. Some ATX PSUs have a single 12V supply internally, others have 2 or more. The label on the PSU should say. If your PSU has 2 or more separate 12V supplies internally, you must not connect them in parallel.
3. If you really want to drive all 4 sections in parallel, then you would need a 56A switch to control them. Your RUMBA board won't handle anything like 56A, so you will need to use a 56A external switch. That in turn would require several mosfets wired in parallel. So instead, I suggest you use 4 external switches, each rated at 14A, all controlled from the same RUMBA output. Then you can connect each heater section to a separate output from your ATX PSU, solving tour wiring problem.
4. If you do run all 4 sections of your heated bed from 12V, that's 674W of power in total. That represents a serious fire risk, for example if the firmware freezes and leaves the heated bed at full power.