Question What version of Shader Graph does Unity 2019.1 use?

and the second question is: can I downgrade it to shader graph 4?

 

1) 5.+
2) No I'm pretty sure you can't, sorry.

 

thanks btw, should I use the one that comes with the package manager or the one that comes with unity?

 

What I do is always use the latest one, because it's pretty stable and comes with improvements and new feature.
So whatever you get with Unity, you can go in package manager and update it to the latest possible version (5.13 on 2019.1 at the time I'm writing this).