I've never used a headstock but personally I would say you will exercise your back more getting them
in to the headstock than you would just walking down the line alongside them, treating them as you go. But, as you only have 4, presumeably they are very tame and perhaps wouldn't be as difficult to get into the headstock as mine would be!
Reading your post again, were you thinking you would do their feet with them stood with their heads held in the headstock? As they're clearly very tame, maybe that would work...
And, reading the other responses, I am now interested to know if anyone's ever used a headstock to restrain a recalcitrant ewe while her lambs suckle?