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.