Quick questions....
Why didn't you buy the 4210 with the folding ROPS? If it wasn't an available option, then why didn't you upgrade to the 4310 which I believe has the foldable ROPS? Getting the folding hinges to do this yourself, cutting the top part of the ROPS off, welding the hinges in place, etc seems problematic. It just seems you're going about this the wrong way. If you knew your trailer couldn't carry the tractor without the foldable ROPS, then why buy it in the first place and then hoping you could solve this problem by modifying the ROPS.
It may be doable but what do you do if it's not? Sell the tractor??????
...Bob