Updown counters

counter of
0000 -> 1111 -> 1110 -> 1010 loop

Make a truth table, with Q1, Q2, Q3, and Q4, and one with Q1+, Q2+, Q3+, Q4+ (the next state)

Q1	Q2	Q3	Q4	Q1+	Q2+	Q3+	Q4+
0	0	0	0	1	1	1	1
1	1	1	1	1	1	1	0
1	1	1	0	1	0	1	0
1	0	1	0	0	0	0	0

This table is a condensed version of the full table from 0000 incrementing to 1111. All the Q1+ etc are X, or Don't Care.

You now have enough info to make 4 K-maps for Q1+, Q2+, Q3+, Q4+. These K-maps will be used to make a combinational logic equation that inputs into Q1, Q2, Q3, and Q4 respectively.