"And notice that Maryland Av. wasn't signed as DE 4 in 1941, and DE 9 didn't exist designationwise, at least in Wilmington."
First post here, native Wilmingtonian (born there 1967). Probably didn't do quoting correctly, sorry about that.
Anyway, my late father was hired by DuPont in 1950 and moved to Wilmington then. He had some old maps that I haven't seen in a while, but I remember this much (what I write below).
At that time, Delaware 4 was what was later Delaware 299 going west of Middletown and ending at the state line just east of Warwick MD. This made a lot more sense to me since it was more like halfway between Delaware 2 in Wilmington-Newark corridor, and the Delaware 6 corridor at the New Castle County/Kent County line (as opposed to now (and the 50 years I can remember back)) where Delaware 2 and 4 are a couple miles from each other, and Delaware 6 is 40(?)+ miles south. I do not remember if Delaware 4 went east of Middletown, but I'm sure some website can answer that question.
Delaware 48 went down what is now Christina Avenue all the way down to what is now the Port of Wilmington -- on the south side of where the Christina River goes into the Delaware River (not far downstream from current I-495 bridge over Christina River). At the end of Delaware 48 was the ferry terminal to Penns Grove. This was one of (if not the only one, not sure about that) the ferries that the Delaware Memorial Bridge (opened 1951) replaced.
I do not know about Delaware 9 at that time.