I would upgrade
The 820 had a steel frame vs. the 4500's aluminum.
The 820 has a 63mm travil fork (if you 820 is old enough it doesnt have a front shock at all) vs the 4500's 100mm travil RockShox Dart 3 with lockout (a very nick for for the price i may add)
Also, you go from a 21 to 24 speed with nicer shifters.
O and much nicer tires and wheeles also
not to mension a couple pounds difference
The 4500 does not come with disk wheeles, however the frame has disc mounts.
Bottem line, the 820 is good for playing arround on, the 4500 is a bike that is respectible even on a xc race course.
I work in a bike shop all day with these things so if you have any questions ask im shure ill know
