For the last 4 gutter downspouts, I chose functional over pretty.
Good call! I too have some downspouts that drain into 4 inch underground pipe. In places a pair of 4 inchers go together into a 6 inch. (note a 6 inch pipe has more than twice the capacity of a 4 inch)
A few of my downspouts are not connected to drain pipe as they drain harmlessly down-slope across concrete, gravel, and grass. All my drain pipes drain to daylight into a creek feeding into a pond or into a pond directly.
The central portion of the house and adjacent sections on either side which contain significant 2nd floor space (My wife's 800 sq ft tea room at one end and my wood shop, metal shop, and 3 car garage at the other end all have relatively large expanses of 12:12 pitch roof. Standard seamless gutter has a pretty good capacity but large roof expanses will absolutely definitely overload it in heavy rainfall.
I have no gutter cleaning-clogging problems and my gutters overflow whenever it rains hard. It is a simple matter of basic arithmetic. The amount of rain falling per unit time per unit area when summed over the area of a large roof feeding a gutter can easily exceed the volume that the gutter can handle without overflowing. My overflow problem is not the capacity of the drain pipes. My gutters can't feed my pipes fast enough to back up the water and fill the downspouts. The problem is the width and depth of the cross section of the gutters (and potentially their placement too.)
Unfortunately too soon old and too late wise!!! A gutter contractor comes on site with a roll of material and forms the gutters in a custom installation that looks nice and seems in line with what you have seen in the past. Unfortunately one size does not fit all. If your buried pipes can handle the volume without backing up and your downspouts can handle the volume that leaves the open question of the gutters themselves being sized to handle the max rainfall you experience. My gutters are "standard size" but are, in fact, seriously undersized for the area of roof they service. Again, one size does not fit all. I suspect you might have this problem as well.
In my case overflow water from the gutters is an inconvenience not a threat to the structural integrity of the house. If that is the case for you then you may elect as I did to grin and bear it. If it is a structural integrity issue then you may need to retrofit at least some of your gutters with larger higher capacity ones. The weakest link of the chain may well be the capacity of the gutters not the downspouts or buried drain pipes.
Best of luck to you in finding a safe and satisfactory solution to the problem.
Pat