On a lever gun, I’d try the leupold 6x36, not the 6x42.
The 6x36 is an FX-2. I’m getting away from a 4x32 FX-2 for better low light performance. If I’ve got my numbers right the 6x36 will actually be darker than the 4x32 given the same glass quality.
^^^^^
Correct (Based SOLELY on the exit pupil)....BUT there is so much more to consider when purchasing a scope. Especially when the purpose is to increase low-light performance.
In the example you've cited above the 6x36 (mathematically) should produce a 6mm exit pupil, the 4x32 an 8mm exit pupil.
Assuming the 'glass quality' is the same (same grind quality, same coatings, same light transmission ratio) then the scope with the larger exit pupil should appear brighter.
But you'll need to consider whether or not YOUR eyes can 'use' the larger exit pupil size and IF the lower magnification of 4x vs. 6x will be a factor for you. The ability to see the object better (larger) and thus pick out details is an important consideration in hunting situations. Only you know the distances and terrain features you normally hunt under.
Back to the importance of exit pupil size. The 4x32 with its 8mm exit pupil is almost certainly MORE light and a larger exit pupil than you can use (unless you're 12 yrs old with perfect, dark adjusted vision). If you are of the age 40 to mid 50's then chances are the pupils of your eyes (dark adjusted) can only dilate to about 5mm.
So if the exit pupil of ANY scope is larger than that... it is of no benefit to you (with respect to brightness alone). You simply can't process it. But...a larger exit pupil CAN be more forgiving in that it provides a larger 'window' to quickly find your quarry through. You won't need to so precisely align your eye.
I think of more importance for you.....will be finding a scope with the best glass quality (highest light transmission rate and best coatings) for the money. Exit pupil size for either of the scopes you cited is moot IMO, both are more than adequate. IF your shots are commonly 100 yds. and in, the 4X scope would be fine. For greater detail or shots 100 yds. and longer I would definitely go with a 6X.
Good luck with your quest.