The brownAND gate is not necessary if this is a stand-alone counter; the output of the A flip flop would be connected directly to JB and KB. The OV output is 1 when the counter is in state 15 (1111). OV could be connected to the JK inputs of another flip flop or, if we built two 4 flip flop circuits like the one above, we could connect the OV output of one to the input where a 1 is now connected to construct an 8-bit counter.