Solved this is a 4-bit synchronous updown counter designed | Chegg.com
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
Synchronous counter
Very Large Scale Integration (VLSI): Synchronous “up/down” Counter