Mike, I think it is a bar of each, meaning 4 beats 1 2 3 4. It does not mean that you neccessarily strum all of those beats when you perform. Your hand keeps moving up and down, but the pick may not make contact every time.
If you are in Stage 1, then keep to strumming 1 2 3 4 down down down down for now, but things get a lot more interesting a bit later on.
Sometimes two chords are played in the same bar, but they will be isolated inside two vertical lines, like on the bridge of Dock of the Bay, in Stage 4. That is the convention in the book for two chords in one bar. Otherwise, one letter pretty much means on bar, ie 4 beats.
I play this using basically the strumming pattern suggested in the book, but I miss the down strum on 3. That comes later.
Hope this helps.