4 196 16 144 36 100 64...............

The pattern is: square the number, then reverse its digits. 4²=16→61, 14²=196→691, 16²=256→256 (reversible). Actually: 4→14 (add 10), 14²=196→reverse=691, 16→square=256→reverse=652. The series alternates between squaring and reversing. The last term 64 suggests 8 was squared (8²=64). This is a pattern-completion question where 64 is the logical next term.