Testing day! Question?

I will be taking a hpt today ! (Well, I've ordered some frer's and they should come today in the post)
​I got up and half asleep I wondered into the bathroom and peed! Naughty me! So now I can't test with fmu! 
​But, I've been holding my smu for nearly 4 hours now, how long should I hold my second morning urine before taking my hpt? AF is due today but no sign of her as of yet..