I'm gonna try to not get carried away and write an essay on reticles. I'll try and answer your specific questions
"T3 at 4x?"
It's doable, although not as reflexive for close yardage as the MPCT series. The MPCT series has the thicker perimeter stadia starting at 8 mil. Whereas if I need to use the tree I feel the advantage goes to T3 because the wind dots double duty as landmarks for quicker orientation at lower mag levels. I have been switching to more T3 because I feel I can orientate my holds faster with T3 at lower mag levels. I felt it was important for me to be able to orientate quickly for the center and to the 1mil mark. This allows me to be faster for anything inside 350. With MPCT 1&2 I found myself lagging too much to orientate quickly (no landmarks for 1mil). Switching to MPCT 3 gave me a landmark at 1mil but as I kept trying to gain FOV with lower mag levels that "see thru" circle got harder to acquire. Hence, I found with T3 I could pickup one mil faster because of the enlarged 4th wind dot serving as a landmark.
"How is it in low light? What mag do you need to run it at to be visible the last few minutes of legal shooting light?"
This is where it gets interesting and the answer varies. I would ask what are your expectations and what type of shooting? And also illuminated reticle or not?
For close range quick acquisition, without illumination, I would say 10x is about where you need to be to pick up against woodline (difficult) backgrounds. Otherwise with illumination 4x is doable (you can find that small red center dot). Overall though not as reflexive for quick and close as MPCT series with thick perimeter stadia and full bright reticle illumination.
T3's illumination is unique and IMO is the best illumination for distance precision. This is because it has bright dots at the center and then every 2mil mark. You can easily orientate, yet the illumination is not there to obscure the target you are trying to focus on. Whereas with many full illuminated reticles, the target gets obscured, even on the lowest settings as the sun goes down.
Here's some pics that hopefully demonstrate what I'm saying.
T3 @ 4x
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T3 @ 10x
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T3 @ 15x
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T3 @ 20x
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T3 @ lowlight & 10x
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MPCT 3 @ lowlight & 10x
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T3 @ lowlight and illuminated 4x
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MPCT 3 @ lowlight and illuminated 4x
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T3 at dark full power illumination
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T3 at dark roughly half power illumination
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Hopefully this helps. If I were near Tampa I'd definitely do a burger & beer. Although would prefer whiskey.