The RAM should be just right for a Dell Inspirion 3521 according to the datasheet. (4GB/DDR3L/1600MHz/PC3-12800)
PC3-12800: 1600MHz x 8 bytes per clock = 12'800 megabytes / second. (This is simply used as a different description instead of DDR3 / 1600MHz). I hope this answer is sufficient.

Addition to the question: PC3-12800