Wheels unevenly awesome–what now?

Hello, I have a problem with my V70 D5 with uneven tires. At the moment I have v.orn left approx. 3.6 , rear left 3.9, front right 5.8 and rear right approx. 6.4mm. My Volvo dealer, who has raised the wheels (for the second season) usually always makes the wheels the same place where they were the year before. Now he thinks that he may have switched back left to front right. My objection, the track could have been misplaced for me. Now I am superior to mounting the better wheels forward and the bad ones backwards, so that I can get out as much as possible, because the front drive is running faster. The dealer thinks that I should just do this the other way around, the better wheels were on the rear axle because of the safety. One could then if a few wheels had gone off t first replace these and later the others. What is better and safer now? Thanks for feedback’s Peter