Should I Follow Up?

I had an interview with a well-known financial services firm last friday for an off-cycle internship, and felt like the interview went well as I got all technical questions correct. However the interviewer mentioned that they will expect to get back within a few days (they will send the decision to HR and HR will contact me), but it's wednesday now and I haven't had a call yet.

Should I send an email to recruiting contact to follow up? Or should I just keep waiting?

(I didn't send a thank you email because I couldn't figure out their email format, but I guess this wouldn't be a factor that prevents me from the job?) 

Any advice will be highly appreciated! Thanks in advance!!