A large arbor for a 4w is not overkill. It all depends on the reel. Cheap inexpensive reels- yes, upper end reels -no. The weight difference is noticable as is the balance.
FWIW, whether or not you get a regular or large arbor, get a good reel.
A good reel with extra spools can outfit different weights of rods. I don't imagine your 4w will be the last rod you'll ever buy.
I have an LRH with 4 spools. 3w,4w, and two (different) 5w lines on them
I fish with cane and glass, and faster graphite rods in 3,4,5w mostly. I have over 20 reels from large arbor to regular in 3w to 8w, and I'd have to say the Hardy is my favorite (it's not so cheap thou) and next is my J.W. Young Condex from the '60's.
Just more fodder to kickaround from the higher end side of it.
(You can probably buy 8 Okuma's or Ross Headstarts for the price of one Hardy, or Abel and they'll probably do you justice, but the Hardy or Abel you'll pass on down the family line and fish it with pride- hopefully)