There might be nothing “wrong” with a fixed 4X, but the 2-7X certainly gives you more flexibility…
The 2-7X gives you a lower power on the bottom end, for close shots, or to help you “find” your quarry thru the scope.
The 2-7X also gives you a higher top end, for shots in those open fields.
I like 2-7X scopes. It’s what I have on my Omega, my .30-06 deer rifle, my .22 “understudy” to my deer rifle, and another .22. Very versatile!
I’ve started putting 3-9X scopes on my more recent squirrel guns, but I prefer a 2-7X on my deer guns.
I always leave my variables on their lowest setting. If something is far enough away that I need to crank it up, I’ll likely have time to do so. If something is close enough (or moving fast enough) that I need to crank it down, chances are decent that I won’t have time to do so!
A fixed scope will typically be more durable (comparable price range, etc.), but scopes have come along way in recent years. Most of mine are the Leupold VX-1 line, or now the “VX Freedom” line.
Decent quality, great (transferable Lifetime) warranty if needed, and name recognition/resale value if you have sell, down the line. They were in the $200 range new for a long time, but the last one I bought was about $260, two years ago. Not sure where they are, now in the current madness!