#log_10{0.0001}##=##log_10(10^-4)=-4#, by definition of the log function. Thus #pH##=##4#, and, necessarily, #pOH=10#.
Note that you should understand #log# functions. When I write #log_aB=c#, I specify that #a^c=B#; thus #log_(10)100=2,log_(10)1000=3#, and #log_(10)10^(-1)=-1#. What are #log_(10)1,000,000#, and #log_(10)10^-6#?