Where S1 and S2 fall in the cardiac cycle and how they correspond with the EKG rhythm. I’ve also included where and why S3 and S4 fall.

S3: dilated ventricle, usually due to heart failure, though it can be normal in children

S4: the sound is caused by blood entering a ventricle with decreased compliance.
A patient with atrial fibrillation can’t have an S4 because there isn’t enough coordinated atrial contraction to force enough blood into the ventricles at once