how do I convert Midi to Audio with Digital performer? (its an older version, version 4.1)

Do you mean record from a midi track with a virtual intrument on it, and then record the audio from the midi track to audio track?

this is what i want to know.

And what about midi from hardware.

yes, if thats how its done, and if its done in digital performer

You need to take the audio outputs of the synth that the midi info is going into, and send it back to a record enabled audio track.

It's on page 180 of the DP manual...
Then you assign a stereo output buss to an audio track (mono or stereo) then record that audio tracks input thats fed from the instrument tracks (Virtual Instruments) outputs... It's a little involved but works really great. Make sure you disable the output (voice) of the instrument tracks (VI's) outputs to conserve DSP resources since you have rendered the VI to audio.

I found some video tutorials on it ... here's a link to your particular question ...
http://www.youtube.com/watch?v=Pe8FwU4IcBo

For what it's worth i've been waiting until i'm completely done with my composition, running only virtual instruments, rewired synths, samplers, and soft synths, plus any real audio tracks, mixing on the go, and then using a nifty utility called "audio hijack" to record a real time version of the playback directly to an aiff file. bingo there's my mix. my virtual instruments rarely go to audio these days, largely thanks to the increased computing power of the new macs.
I track in DP with my FX plug ins off, low buffer settings for good controller feel, and then bump up the buffer, turn on my power hog plug ins, and mix away. a real time saver for me, it encourages a certain type of work flow, of course, but i'm frequently in a crunch, and sort of enjoy mixing as i go.
my two cents.

I do that too if I don't have to worry about high track counts because playing an audio track is trivial compared to a virtual instrument triggered by midi... sometimes it's best to freeze or render the VI as audio. I know I can play back @ least 100 tracks of audio... I know for a FACT I can't do that with VI's so rendering to audio is the best thing (if your only using 1 machine) now if you have 2 well then time to rip!