Both are great.
Do you have issues with hip belts cutting off your hip flexors?
I know a number of guys that hike uphill with their 4500 waist belt unbuckled because it cuts the blood off to the thighs, myself included.
The 4500 is almost 5000ci, but the Sitka 45 is right at 4500ci.
If I were doing it all again I would get a Sitka45 based on one good friend's review of the 45 after about 20 days in the backcountry in August and Sept.
FYI he sold his 2008 4500, his second one, to my father.
He and I are both smaller framed guys, I can hike in with a weeks worth of gear and food and have less than 200lbs on my boots so the issues that make the Sitka possibly better for my buddy and I may not fit you if you are larger framed.
I will say Badlands has an incredible warranty. I am only 5'7" but have extremely high hips, aka short torso. After many problems Badlands built me a new pack from the ground up and it is ten times better than the stock Medium pack, but I still get numb hip flexors on steep uphill climbs.
good luck