Make sure to read the release notes for the first, second and third release candidates!
This fourth (and hopefully last) release candidate is the product of an audit performed by LevelK. This audit increased our confidence on the code that we are about to release as version 2.0.0, and allowed us to add many improvements and a few bug fixes that will make this the best OpenZeppelin release so far.
In particular, we decided to remove the BreakInvariantBounty contract because of a front-running issue (#1333) that will require a heavy redesign, we added the nonReentrant safeguard for buyTokens in the Crowdsale contract (#1438), we improved the SafeERC20 allowance handling(#1407), on TimedCrowdsales the closing time now must be strictly after the opening time (#1440), we fixed how AllowanceCrowdsale checks remaining tokens (#1449), we fixed a reentrancy issue on FinalizableCrowdsale (#1447), and improved the ERC721 implementation (#1450).
There were many more low issues and notes fixed. Check out the LevelK Audit - OpenZeppelin 2.0 project for all the details. There are a few minor issues still open that we are going to leave for the 2.1.0 release.
We want to thank LevelK and all our community contributors that have joined us on this long release candidate journey. The full list of people who have helped us will be in the final release notes, which is next!