4 bit Asynchronous Counter with J K Flip Flop - YouSpice
4 bits Synchronous Counter with J K Flip Flop - YouSpice
How to design a synchronous counter 4 bit using JK flip flop that can count up even numbers from 0 to 14, and count down odd numbers from 15 to 0 in 1 system - Quora
Basic Tutorial Lesson 11: Building a Binary Counter Using JK Flip-Flops - Emagtech Wiki
Synchronous Counter and the 4-bit Synchronous Counter