Technical details for this first release of ILSpy 4.0 are available in a dedicated page in our wiki: SRM deals at length with information that is important to addin writers, the background story and some of the performance/memory improvements you as a user of ILSpy can expect.
Our goal for this version given the underlying architectual changes is as follows: "As good or better as ILSpy 3.2 in terms of decompilation quality, stability, performance and memory usage."
Therefore we ask you to compare the last released production version with this Alpha 1 - we are confident that even at this early stage we match the quality in decompilation. If not, please do open an issue (sample assemblies very much appreciated!)