I think you are making this more difficult than it really is....
the four is insignificant (because 0/4 is still 0). so now we have:
I think the 4 is confusing you so let x=4θ so now we have:
and the solutions to that is when x=pi/2 or 3pi/2
now substitute 4θ in for x and you have
4θ=pi/2 and 4θ=3pi/2
so dividing by four yields:
θ=pi/8 and θ=3pi/8
Hope that helped!!