You gotta give it up to the finder. If you're able to identify this sample from only hearing the song, which most of us probably have heard 100 times on the radio, than you have a blessed ear.
Since Im interested in trivia, I always try to understand why producers decide to combine certain songs, as in this case Thelma Houston's 'Don't Leave Me This Way' and the Commodores 'Three Times A Lady'. Since I myself have been playing around with samples, making beats, I know that its not uncommon that the reason why I have chosen to combine certain samples is simply that they appear together somewhere.
Applying that assumption, I went looking for compilations where these two songs appear together, and just as I expected they both appear on this one.
http://www.discogs.com/Various-25-1-...elease/3406822
Even though this doesn't add a lot of knowledge to this specific track, it perhaps tells us something about the process of making this epic beat.
Another example, also involving 4th, is the Wu-Renegade track, which combines the piano from Christian Sinding's 'Rustle of Spring' and the string from Nikolai Rimsky-Korsakov 'The Sea and Sinbad's Ship', which both appear on this compilation, right after each other.
http://www.allmusic.com/album/great-...s-mw0001847770
This method of finding samples should be applicable in other cases as well, especially when we're dealing with samples taken from previous billboard hits.