I'm no expert but do own decks and yes there are four beats in a bar and you need to bring the other song in at the start of a bar BUT most songs are 4:4 which means they have four bars made up of four beats. So on every sixteenth beat is where you should bring in the other song (4x4). Learn to count to 16 and you'll be sorted.
This was just totally obvious to me, but hopfully that helps....
Have fun
This was just totally obvious to me, but hopfully that helps....
Have fun

Trusting your ears is usually the best measure (boom boom), don't think in seconds, concentrate on the beats.