A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 4820256 can be evenly divided by 1 2 3 4 6 7 8 9 12 14 16 18 21 24 27 28 32 36 42 48 54 56 63 72 84 96 108 112 126 144 168 189 216 224 252 288 336 378 432 504 672 756 797 864 1,008 1,512 1,594 2,016 2,391 3,024 3,188 4,782 5,579 6,048 6,376 7,173 9,564 11,158 12,752 14,346 16,737 19,128 21,519 22,316 25,504 28,692 33,474 38,256 43,038 44,632 50,211 57,384 66,948 76,512 86,076 89,264 100,422 114,768 133,896 150,633 172,152 178,528 200,844 229,536 267,792 301,266 344,304 401,688 535,584 602,532 688,608 803,376 1,205,064 1,606,752 2,410,128 and 4,820,256, with no remainder.
Since 4,820,256 cannot be divided by just 1 and 4,820,256, it is not a prime number.