A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 4721376 can be evenly divided by 1 2 3 4 6 8 11 12 16 17 22 24 32 33 34 44 48 51 66 68 88 96 102 132 136 176 187 204 263 264 272 352 374 408 526 528 544 561 748 789 816 1,052 1,056 1,122 1,496 1,578 1,632 2,104 2,244 2,893 2,992 3,156 4,208 4,471 4,488 5,786 5,984 6,312 8,416 8,679 8,942 8,976 11,572 12,624 13,413 17,358 17,884 17,952 23,144 25,248 26,826 34,716 35,768 46,288 49,181 53,652 69,432 71,536 92,576 98,362 107,304 138,864 143,072 147,543 196,724 214,608 277,728 295,086 393,448 429,216 590,172 786,896 1,180,344 1,573,792 2,360,688 and 4,721,376, with no remainder.
Since 4,721,376 cannot be divided by just 1 and 4,721,376, it is not a prime number.