3:14 compare to 3.14 an approximation of pi.
π ≈ 3.1415926535897932384626433832795028841971

This number is the ratio of the circumference of a circle to its diameter.

4:20 is frequently referenced in cannabis culture as time to smoke. It's also 42*10, and 42 is the meaning of life. (according to hitchhiker's guide to the galaxy)