You may want to look at the higher baseboard given the ceiling height. Just FYI, there are several resources that would note the height of baseboard vs ceiling height if you are following a more traditional proportion system to use as a starting point-

You don't have to use the profiles show-I have had luck with simpler beads and reveals for a modern look, but the proportion of the pieces to the whole is a key part-for example the 5 1/2" tall trim is broken down to have about a 1 part cap moulding to a 4 part base board.

Thanks. We are choosing a very simple profile.

Here is the simple profile we want, with no cap

We install 5 1/4" base in homes with 9' ceilings. That's the height derived from classical proportions. It's definitely more pleasing to the eye.

We have 9' ceilings in our home and 5 1/4" baseboards. It looks more proportional.

How much space is there under the window after its trimmed out?

51/4 inch baseboard, 9 ft ceiling

9' ceilings on the second floor and we have 7" baseboards (actual size)

My ceilings are 12’ but I have 7” baseboards in your profile and they are beautiful.

I agree with others, definitely go with the 5-1/4". You will regret the 4", IMO, with 9' ceilings.

I agree with everyone voting 5 1/4" base boards with a 9' ceiling. Will be so much better visually than 4"

My baseboards look exactly like yours, with quarter-round at the base, and they are 7 1/2" with 9' ceilings.

Yes, you will regret it. We used 5.25" in areas with 9' ceilings (and even 8' upstairs) and used 7.25" in areas with 10' ceilings. I can't imagine using less than 5.25".

I'd use larger trim around doors and windows too!