I haven’t tested with 4.0 specifically (it’s on my TODO list), but we did add pretty good Multisite support last year, and I’m not aware of any core WordPress changes that would cause incompatibility. If you try it out and find issues, I’ll be happy to fix them.
I actually released this plugin a year or two before SendGrid released their official one. The main difference between this and the official plugin is that this one tries really hard to be compatible with as many WordPress installs as possible out of the box. You don’t have to install another Mail plugin, and you don’t have to have CURL on the server (because we use the WordPress HTTP API).
Obviously I’m a little biased, but I feel that this plugin is a higher quality WordPress plugin than the official one, which is why it still exists. If you find anywhere that mine’s worse than the official plugin, please let me know!