Actually my question would be how big of a gap in term of precision between internal oscillator and a resonator at 4 MHz?

I am wondering if I can just remove the external part, save some space and the cost of the part.
I don't need a long term calculation such as an alarm clock, I am just calculating distance using ultrasonic and I don't need precise distances. I only need to compare distance differences in units as long as during my comparison my units are identical.

Mike