When should I test? What brand should I buy?

Aunt Flo is 4 days late now. I've been regular and the <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> has been spot on since January due to treatment. I have a pituitary tumor. I took one the day I missed my period and it was a BFN, but it was a really cheap dollar store one. What should I do?!? I'm so anxious! Don't want to be let down.