There is no way a 2 ohm 83db sensitivity on 100w is going to get up to 94db sensitive with 50w, if all else is equal. I would also be leary about a 94db sensitive midbass....that is really high sensitivity and I would personally want to investigate how that number was determined. Similar with the 83db sensitivity, how was that determined? The only way that I have seen that is semi-accurate for this is using something like the klippel to measure the speaker and then actually see the response and look at what spl it is at. Otherwise it seems the numbers are usually off by quite a bit.
If the 4ohm was rated at 1000hz, it might be only 80db sensitive in the sub frequencies. Also is it 2.83v/m or 1w/m ratings. In other words, too many variables to actually be able to figure this out. Do you have Frequency response charts of both speakers? That would be better help than just a sensitivity number.
If all things are equal though, you can see below the power to achieve certain SPL's assuming no compression.
2 0hm 83db sensitive
1w @83db
2w @86db
4w @89db
8w @92db
16w @ 95db
32w @ 98db
64w @ 101db
100w @ ~102db
128w @ 104db
4 0hm 94db sensitive
1w @ 94db
2w @ 97db
4w @ 100db
8w @ 103db
16w @ 106db
32w @ 109db
50w @~110db
64w @ 112db