The Sims 5: We'll be able to convert ts4 content to ts5?

A bit too soon, yeah. Ts5 is speculation at this point. For the moment they don't seem to be working on Sims 5, but on project Renee, whatever it is - and even that one doesn't seem to be at a stage where this question can be answered.

However, if the game uses 3D models and some sort of picture-based textures (maybe similar to the bases TS3 uses if it's some sort of color/pattern drag-and-drop), then it's likely someone would make a program to extract resources from the game, like for the other Sims games.

Probably better to ask again if/when the game comes out.