assuming i'm in 4/4 and the melody starts on beat 4 of a bar - so that's a pick up bar - do i count the first beat as beat 1 of the next bar or backtrack to have three beats silence and set bar 1 on beat 1 of the pickup bar?

in case i've not made it clear think of 'O shenadoa I love your daughter'

the O is beat 4 pick up and the melody starts properly on beat 1 with shenendoah..............

apologies to proper musicians i'm sure there are proper terms for what i'm talking abvout