What matters is fit, fit , and fit. How the pack fits you, how the pack fits your gear, and how the pack fits you with your gear.

It is you and your gear that matter. Many people recommend boxing up your stuff and going to the store during non-busy hours and put your stuff in the pack.

I will say, in general, a 4000 in3 pack is on the large size for lightweight backpacking. However, if you have a big synthetic sleeping bag, it would take up most of that volume.

In my opinion, the downside to having extra space, is humans tend to fill it. That is one significant way you get weight creep. I would not recommend getting a pack to fit gear you might get in the future.