When it is plain weave it doesn't matter if the warp has an even # of warp threads or an odd # of warp threads and it doesn't matter if it is divisible by 4.

When you thread 1,2,3,4..... and no matter where you end up, on a 1, 2, 3, or a 4....and if your tie-up is 1 and 3 against 2 and 4...every single warp thread is right beside each other so every other one will either go up or down and voila....you have plain weave.

I often warp with an odd number of threads in my warp because I use odd numbers from the fibonacci sequences 1 ,3,5 which will all give you odd numbers in your warp and it is absolutely 100% okey dokey :)