Try playing bass then treble,four times, as if you were in 4 /4 then plucking on first beat simultaneously two notes,when you have acheived this it will take some time
then try reversing this,so that you play a half beat with finger followed by thumb, your aim should be to play thumb then treble or treble then thumb, or both together, then you will be on route to melody picking in 4/4while keeping a bass going