Planet x has 4 times the mass of earth and a radius of half that of earth, how many times larger is the acceleration due to gravity on planet x then on earth?

4 times the mass of Earth . . . multiplies Earth gravity by 4

1/2 of Earth's radius . . . multiplies Earth's gravity by 4

both changes . . . multiplies Earth gravity by (4 x 4) = 16

The acceleration of gravity on the surface of Planet-X is 16 TIMES that on the Earth's surface ... about 6.3 times what it is on Jupiter.

You could not walk around on Planet-X, and it's pretty likely that whatever contraption you landed in could not take off again.  You're stuck on Planet-X, living out the short remainder of your life just laying there like a heavy pancake and using all of your strength just trying to breathe.  It won't be pretty.