Will it be enough to just be watercooling my cpu and gpu with a 4x120 rad and a
2x120 rad without having ANY other fans in my case. As in I will only have the 6 fans for the radiators and thats it. Or do I still need add extra fans around the case to be air cooling?
The case will be a corsair 900d. Also, is the 4x120 rad and the 2x120 rad overkill for just cooling cpu and single gpu, even if i highly overclock?
That should be sufficient to cool both components. If anything that may be a bit too much radiator. You'd probably be better off with two 2x120 rads and 8 fans in push/pull config. Just be sure to have the rads in between each component, ie CPU -> rad -> GPU -> rad -> CPU. This will help to more evenly distribute the heat and keep each component cooler.
It's certainly good to have a few fans in the case, if nothing else to pull some cool air in so you're not pushing hot air through the rads, or to push the air out if you'll be pulling air in through the rads so your ambient temps don't go through the roof.