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In nearly every PSO forum ive been in theres people saying "pso is the follow on to ps4" and "pso is set before phantasy star 1" blah blah blah.
ive found a timeline wich a site has done, listing the storylines of PS 1-4
and the dates, characters and big events

http://www.phantasy-star.net/psu/psutime.html

if you have an hour to burn then this is a very good read, and it shows just how deep the phantasy star storyline is.

Enjoy...