How long to follow-up?

I just recently started looking for my first full-time engineering position, and was wondering how long I should wait to follow up on leads.

Recently a startup I'm interested in contacted me through AngelList to talk about a position, so I quickly expressed my interest and the CEO got back to me pretty quickly. Unfortunately at the time I couldn't get on the phone to talk as he/she wanted to, so I told him that I would email him back when I had some more free time. Two days go by and I finish up a client project so I email him back to have that conversation.

It's been almost 4 business days and I haven't heard back. I'm hoping it just got buried in his inbox since I sent the email at 2AM, but am unsure. Should I follow up now, or wait longer to avoid looking creepy?