What annual rate turned 8,000 into 12,500 over 4 years?

Based on these numbers, the CAGR (compound annual growth rate) is about 11.80% per year.

4 years

What is CAGR?

CAGR (Compound Annual Growth Rate) is the constant annual rate at which a single investment would need to grow to go from its beginning value to its ending value over a given period — it assumes one entry and one exit, unlike XIRR which handles multiple cash flows.