R 3.4.0 (upon which MRO 3.4.0 is based) is a major update to the R language, with many fixes and improvements. Most notably, R 3.4.0 introduces a just-in-time (JIT) compiler to improve performance of the scripts and functions that you write. There have been a few minor tweaks to the language itself, but in general functions and packages written for R 3.3.x should work the same in R 3.4.0. As usual, MRO points to a fixed CRAN snapshot from May 1 2017, but you can use the built-in checkpoint package to access packages from an earlier date (for compatibility) or a later date (to access new and updated packages).
The version of Microsoft R Server shipping with SQL Server 2017 will still be based on 3.3.3, but I’m going to guess that a new version of Microsoft R Server supporting 3.4.0 will ship in the next several months.