1% increase in hematocrit with no increase in blood volume, thus a 1% increase in "global hemoglobin" (it's difficult/expensive to measure so not viable for most of us), will net ~ .62% increase in aerobic power output, which in turn will net an approximate .38% increase in speed at aerobic-dominant pace
some things to consider with that info
an event like the 1500 taps into a considerable amount of power through the anaerobic process (can be over 40% for anaerobic monsters, closer to 20% for anaerobically challenged), and epo even before it has time to make more rbcs has shown that it helps shuttle lactate and/or allow one to sustain higher lactate levels without "tying up", but generally speaking one can still get a close approximation assuming it won't increase aerobic power (even though it actually does somewhat)
i recommend making your calculations in 1% increments (yay for exponents and calculators), meaning you'll get an erroneous estimate on the high-side if you use the formula in paragraph 1 to estimate in the following way:
40% hct --> 50% hct = 25% increase (no, it's not a 10% increase, 10/40 = 25%)
25% x .62 = 15.5%
15.5% x .62 = 9.6% faster for aerobic exclusive event like marathon (not scientifically correct terminology or the actual truth, lactate is always contributing, but for sake of making calculation this is effective) thus a 2:30 marathon turns into = wrong
going from 40% hct --> 50% in 1% increase increments (thus 40 to 40.4 to 40.804 etc) will take twenty-two 1% jumps up from 40 (40 x 1.01^22 = 49.78, 40 x 1.01^23 = 50.29)
corresponding highest potential increase in speed = 1.0038^22 = 1.087, thus a 2:30 marathon at best could turn into ~2:17
of course we know that a clean 2:12 runner is not going to suddenly run 2:00-2:01, so what gives? well, for starters how fast we can run is not just due to oxygen-carrying capacity, absolute speed (flying 150-200 speed is a good metric) matters, tendon elasticity matters, other things matter, but i find it very hard to quantify and i think my experience and knowledge with the substance has led me to believe to some extent (of course he can't completely tell the truth don't be naive) that Renato saying the best of the best east africans (many many generations of ancestors lived at altitude in generally warm weather with very consistent moderately high UV exposure year-round) simply do not benefit (as much) as people who come from sea-level-ish ancestors
so as one gets faster, i would say it is indeed true it helps them less by %, but there are such small gaps among the very best that a small % can make a drastic difference so it's still a game changer
anyways, using the OPs initial example of a 4:30 miler, and lets say it's a white dude born and raised at sea-level just as his ancestors were
lets go with an aerobic : anaerobic power output split of 67% (x = .67) : 33% (y = .33), a base hematocrit of 40% with a target hct of 50%
so that is twenty-two 1% jumps, netting an increase in aerobic power output by about 13% (22% x .62), thus the aerobic portion (x ) that was 67% has increased to 1.13x equivalent to .67 x 1.13 = .75, then add the .33 in and we are at 1.08 meaning 8% more power than before. 8% more power will net about 5% more speed
soooo 4:30 --> ~ 4:17 at best, half that improvement at worst, so range of 4:17 - 4:23/4
there are of course other factors, especially psychological like the potential of taking training, diet, recovery much more seriously when using because "now you're being serious and want to maximize gains and make it worth it", and doing the same in craces
who is helped most by increasing hematocrit (again, global hemoglobin for you sticklers)? people born, raised and descended from sea-level ancestors, with a lot of basic/max/raw speed reserve in the tank (like a 4:30 miler that can run a 23-flat 200 as opposed to 4:30 miler with 26-flat 200 speed) and an aerobic system that has not been fully maximized (either short-term in your own single life, or long-term in the many lives of your ancestors due to living at sea-level and doing viking stuff with swords and boats instead of running around and generally trying to not overheat or die of thirst/starvation)
it's not super complicated, but generally this is all just sitting in my head and i don't have discussions in words with myself about it so am not used to writing it all out thus it comes across a bit convoluted surely
TLDR: can't properly estimate the OPs examples of 4:30 or 4:10 miler because i can't accurately calculate their aerobic:anaerobic power output ratio without another pb being provided and i don't know how much absolute speed reserve they are working with or what their starting hct is and how high they are willing to go (still lots of room for improvement beyond 50%, can't recommend going above the upper 50s though it gets a little dicey especially if you sweat profusely)
anyway, feel free to ama, i promise i know more about this than over 99% of the forum (and nobody ever lies on here so it's legit for sure for sure)