The TM M4 SOPMOD recoil engine is very new system that just came out. Normal spring won't work on them and you need to modify them to fit plus I don't think this would be able to hand the M120 spring easily in stock form.
It also need a special battery and charge, regular battery won't work in there.
Here is the link of battery and charger,
I suggest to keep that gun in stock form until a lots of after market coming out.
That gun is not noob friendly if you don't know basic AEG repair, it need a lots of modify to run smoothly or otherwise just keep stock form. It would last for long term.
Try to use google search for TM M4 SOPMOD, there are at two or three review and mod on them. I suggest you to read them.
Edit : Normal M4 magazines won't work on them as well, it must be TM M4 SOPMOD M4 magazines.