> Candidate would be compensated, obviously. That's why it's expensive

Ok... take me through it. I apply to your company and after a short call you offer me to spend 4 weeks working at your place instead of an interview.

I go back to my employer, give them resignation letter, work the rest of my notice period (2 months - 3 months), working on all handovers, saying goodbyes.

Unless the idea is to compensate me for the risk (I guess at least 6 months salary, probably more), then I do not see how you'd get anyone who is just a poor candidate to sign up for this.

> You don't need him to become efficient

So what will you see? Efficiency, being independent and being a good team player are the main things that are difficult to test during a regular interview.