I'm currently porting it to OS4. Don't know how long it takes, there are some small differences in using libraries. But I know that I love CodeBench 
Hopefully HTMLView works in a native version, is there any OS4 software using it?
This might be a problem. AFAIK the current (last) version of HTMLview.os4 doesn't run and seems to be severe broken. The 68k (OS3.x) version doesn't behave really better. I tried to test your 68k (OS3.x) build with OS4.1 - no way, it crashes unrecoverable while trying to start/use HTMLview...
