If you are moving from version 10.2 or 10.3 to 10.4, your transition should be fairly smooth. Although 10.4 contains lots of new features, the basic architecture of the OS has remained relatively stable, so you shouldn't have problems with applications not working (at least not with those that worked under 10.3).
If you are upgrading from version 10.1 of Mac OS X to version 10.4, the upgrade involves a lot more than the change in number from 10.1 to 10.4 would indicate. There are major architecture changes from that early version of the OS, and moving to 10.4 requires some thought. It might take some time and effort to get into version 10.4's world, but the benefits are well worth the work. The move from 10.1 to 10.2 made extensive changes to some fundamental aspects of the OS and consequently, many applications that worked fine under 10.1 do not work under 10.2 or later without being updated. The good news is that it has been a couple of years since 10.2 was released, so most of these applications have been updated by now. However, be aware that if you are currently running 10.1, you should check for updates to the applications you use if you haven't updated them recently.