TTC #1 - digital OPKs HELP!?

Laura • 27, pregnant with baby girl #1, due 25th January. So excited :D
I have 4 clearblue digital ovulation test sticks left, I'm pretty sure I ovulate day 18 of my 31 day cycle. Can I just use those 4 tests to confirm or does it need 10 to get a 'build up' of hormone or something? Help?