let us take a 2 bit binary counter, it gives counting as 00,01,10,11, ...so this output is given as input to 2*4 decoder, where we will be having 4 output lines, which only one will be active at any time, based on the input we get,
input to decoder output of decoder
00 1000
01 0100
10 0010
11 0001
this is same as ring counter counting 4 states,