If I sleep more time in stage 4 or in rem, do I get the same brain rest while sleeping fewer hours? If I workout before bed, do I hit stage 4 sooner?

No good answer. Rem sleep is about 10 minutes in the roughly 90 minute sleep cycle and appears to be the most important part .Yes it varies a little from person to person. The only way to know the answer to your question is a sleep lab. One clue---- you dream in rem sleep. The best sleep time for most people is 6 to 8 hours---period.