How much power loss in the coaxial cable?
M17/113-RG316 is not sweep-tested higher than 3 GHz. It does not have guaranteed characteristics at 4 GHz. The extrapolated insertion loss is about 56 dB / 100 ft at 4 GHz (~3 dB at 1.5 m).

To reduce measurement error, check your terminations, calibration, and make sure that the cable is not bent.

Do you have access to a network analyzer with proper test cables?