On Wednesday, 7/27, the Exchange Sustained Engineering team re-released Exchange 2010 SP1 RU4. This release is being tracked by KB 2579150 and will contain a download file that is unique from the previous release. This updated rollup contains all of the changes that were in the original rollup with the exception of the regression because if which the previous RU4 was pulled back.
For customers who have the previous version of RU4 and the IU 2581545 installed, there is technically no need to install this updated version of RU4 as the combination of those two releases is functionally equivalent to the updated RU4 dated Wednesday, 7/27. The only thing a customer would gain by installing the updated rollup 4 would be performing the IU maintenance now that they would need to perform later when installing RU5.