Well the Tianhe-2 core module is in storage and all ready to go, it was built together with Tianhe-1 as a backup should the launch go wrong and since it wasn't required it's available for whatever mission you can think of immediately.

Lab 3 and lab 4 do not exist yet, so they will need to be built first. CNSA has expressed interest that they welcome other countries if they want to be a partner and build a module of their own to attach to Tianhe-2. Many years ago Italy expressed interest and came up with a module design called HERD (High Energy cosmic-Ray Detection) but that's probably dead now.

Here's a rough idea of what phase 2 expansion will look like. Obviously lab 3 and 4 are just placeholders and clones of Mengtian and Wentian without the solar panels. Note during phase 2 expansion Tianhe-1's solar panels will be detached and moved to the end of Mengtian and Wentian.