~ap4.tmp is part of a video codec for .mpg files.  This codec belongs to Microsoft Corporation and is used with Windows Media Player.

~ap4.tmp is unlikely to cause problems unless it has been corrupted by malware such as viruses or spyware programs.  Registry files such as this one can remain damaged long after the offending software is removed.  It’s even possible that the software can name one of its own components ~ap4.tmp in hopes of fooling the host computer.

The best way to check and see if this kind of file is outdated, damaged, or falsified is to run a registry scanning/cleaning program.  There are plenty available on the market today.  One of the few that will scan your computer for free is called ZookaWare PC Cleaner.  It can be found at regzooka.com.