elveax:
But if I've got an ultrasonic sensor that requires 5v and I provide it with 4.5v, won't I run into the same problems I did when I provided it 3.3v?
I don't know. 4.5v is an awful lot closer to 5v than is 3.3v. If it was my problem I would certainly do some experiments to find out if it would work. A 5v Uno or Mega works fine on 3 x AA cells.
...R