OS4 has a built in 68K Just In Time Compiler. This doesn't put all older applications in a "sandbox". Instead, only the application itself is recompiled, and it then interacts with the new libraries etc. This won't provide as much compatibility, but it means that you can use older apps just as easily as new ones - there is no difference to the system.
The big problem is of course the custom chips. Any applications that bang on the hardware simply won't work. One solution is to connect the A1 to an A1200 to reach the AGA chips, but the company doing that seems to have got very quiet. The other way would be a full port of UAE to OS4 - doesn't exist yet.
The older the app is, the less likely it will work. Newer apps supporting RTG and AHI should go fine, while some really old games, and virtually all demos, won't work at all.
