I know this thread is old but I am going to add that I have very recently built and used systems with both the Define R4 and 550D. Keep in mind these are systems with Gigabyte Z87X-UD4H, 4770K, 2x8GB, and single GTX 760 card. One has Samsung SSD w/ 2x2TB RAID 1 array and other is Samsung SSD w/ 1 1TB drive. Both have 2 intake fans up front and one exhaust fan in rear.
Both are great cases. They are both similarly built. I think the 550D has an edge on build quality. Here are some observations
Cable routing is good with both cases
USB on top surface of R4 is great but not sure about dust forming over time. 550D is ok with 2 top facing front.
550D has annoying white LEDs for power and HDD activity. I have tape over mine...
R4 has annoying blue LEDs and they are a little more difficult to cover up with tape.
550D's bottom feet slide around nicely on hard floors. R4's are stick and case will not slide at all. Squeaks and catches. Not good for my tastes.
R4 door is nice and solid, easy to open if you grab the catch easily but only opens one way. 550D door opens either way but you need some force to open and close but I actually prefer it.
Fans and cooling on both is great. I do like the 550D optional popout cover on the top vs R4's screwed in foam covers.
side panels on 550s are ingenious. screw on doors of R4 are no different than some from 15 years ago. They are ok but I love the 550D's panels.
Both are equally quiet and seem to cool similarly as well.
The R4 is a very wide case imo. The 550D is deeper and taller. I'm ok with either but they both are really big though there are bigger ones.
Oh, the R4's panels with cutout holes for fans aren't structural rigid. This makes sense, but they will warp or "pop" under some circumstances. This does not happen with normal use and is not an issue in any way but something to note.
I would be ok with either case but for me I prefer the 550D by a hair. I do like the USBs on the top surface of the R4 and some other touches too so it is close. The little things do it for me, and those little things may mean something different for you.
Given the R4 costs a little less right now that may matter to you. Otherwise either one is great.