I've been meaning to try this for ages but never got around to it.
I think it's desirable in this case to use the genuine Dolby decoding process, and not any old generic upmix algorithm.
But AFAIK there is no simple way to get a Dolby Prologic upmix in software. The methods I've seen on Doom9 (actually for DPL II which gives a 5.1 upmix, but the same method should also be applicable for a DPL 4.0 upmix) use the audio decoder filter present in WinDVD or PowerDVD, inserted into a filter chain in Graphedit, with the output directed to a set of WAV files.
Guides here:
http://forum.doom9.org/showthread.php?s=&threadid=57736
http://forum.doom9.org/showthread.php?s=&threadid=83384