I think that given you're already most of the way through your training block, just stick with what you have planned. What I'd say is that you need to get the time in zone, so start out each interval as planned at 120% FTP, but if you are unable to hold that power through the 4 minutes, then just allow your power to fade (preferably not below 105%), as long you keep going through the 4 minutes. Imagine it being like a badly paced 4 minute hill climb TT!
As it happens, there's some good research regarding the benefits of decreasing power intervals, but they tend to utilize a much harder start in order to reach VO2 Max sooner.
EDIT: just seen @dubtap's post, we have the same source (which I think he posted previously on this thread)!