Differential diagnosis
S3 can be confused with the opening snap of mitral valve in mitral stenosis. The opening snap is higher in pitch and is usually occurring earlier in diastole.

S4 can be confused with a split S1 or a S1 followed by an ejection sound. S4 is of lower pitch than the other sounds, and an established timing by EKG or by palpation should be enough to distinguish these sounds.