I tried to wrote verilog code of 4-bit register with parallel load. But i failed. I am pasting code with output.Please identify whats wrong with it
It looks like you accidentally simulated the four-bit register itself instead of the testbench, so it's as if there was no stimulus and the register output remains undefined (but not high impedance, as you said).

Another problem could be a missing timespec.

Please format your code properly if you expect more detailed help.