S3, S4:
S3&S4 are lower frequency heart sounds compared with S1&S2. Therefore, it's best heard with the bell of the stethoscope at the apex when pt on the left side in order to bring the heart closer to the surface.

S3 occurs when the chordae tendonae are so tense due to rapid fluid overload filling during diastole. The tense chordae tendonae causes the S3 sound right after the S2 sound (the closure of the semilunar valves).

S3 is normal in children and athletes. It simply means the ventricle is capable of expanding and get more fluid.

S3 is pathologic in middle-aged and older people because it signifies fluid overload (CHF)


S4 occurs when the atrium tries to fill the thick-walled ventricle. This is always pathologic.







References:
S3:
https://books.google.com/books?id=0lxSGJYeXikC&pg=PA36&dq=s3+chordae+tendineae&hl=en&sa=X&ved=0ahUKEwiam5nrt5_SAhVBWmMKHcsHDcIQ6AEIKTAC#v=onepage&q=s3%20chordae%20tendineae&f=false

https://books.google.com/books?id=iAijBQAAQBAJ&pg=PA48&dq=s3+chordae+tendineae&hl=en&sa=X&ved=0ahUKEwiam5nrt5_SAhVBWmMKHcsHDcIQ6AEIMzAE#v=onepage&q=s3%20chordae%20tendineae&f=false