Has to be Montell Griffin, December 1996 through March 1997. enters a highly rated 25-0 prospect/contender, with a victory over James Toney already under his belt. takes an even wider decision in the rematch with Toney, at this point 53-2-2 (35), with his only defeats coming against Griffin himself and unbeaten Roy Jones, Jr. picks up a win (albeit via DQ, a win is a win) over Jones himself, then standing at 34-0 (30). It was all downhill from there, but damn what a pair of W's to have (no matter the spacing across your career, let alone back-to-back!)
I guess you have to give honorable mention to Harry Greb, for kicking off 1919 (January through April) with a run of 19-0 (2) that included victories over Levinsky x2, Houck x3, Brennan x2, Fisher x2, Wiggins x2 and Miske. Still not sure, even allowing for the impressiveness of keeping such a rigorous schedule, as well as taking contemporary writers at their word that all of these were fine fighters for their time, that that all quite trumps peak Toney & peak RJJ back-to-back, though.
Best four year stretch: Jimmy Bivins 1942-46. He beat Ezzard Charles, Archie Moore, Joey Maxim, Lloyd Marshall, Anton Christoforidis, Lee Savold, Bob Pastor, and Melio Bettina. Between Oct 42-Feb. 43 he fights Pastor, Savold, Charles, and Christoforidis. Not bad.
I was just looking up Emile Griffith and between Aug. 1960-Jun'61 he beat Florentino Fernandez, Luis Manuel Rodriguez, Benny Paret, and Gaspar Ortega. 'Tween Aug. '33-Jan.'34 Barney Ross beat Tony Canzoneri, Sammy Fuller, and Billy Petrolle. Twixt March and November Henry Armstrong beats Barney Ross, Baby Arizmendi, Lou Ambers, and Ceferino Garcia. I know that two of those aren't quite within the parameters you're looking for but it's still damn impressive.
Toney looked a lot slower against Griffin than he did against McCallum and did much better against the bodysnatcher than a fighter on Griffin`s level would, Toney was out of shape in their first fight and hardly any punches were thrown in their second, Jones was awful in the first Griffin fight and it proved if you jabbed well against Jones you could get him against the ropes and he didn`t know how to escape.
Toney was not as his peak at super middle and Jones proved Griffin wasn`t that good by knocking him out easily in the rematch.