As you probably have known, i have bought a new desktop and laptop, both equipped with 4 GB of RAM, but i'm still using 32bit Slackware Linux, so there's a limitation from the OS to correctly detect 4 GB of RAM. In order to solve that, in most article says that we have to start using 64bit version.
That is the easy solution, but what happened if you are too lazy to reinstall and reconfigure everything? Well, if your processor support PAE, then you are quite lucky, since you only need to recompile your kernel and enable HIGHMEM64G config and it will try to enable PAE extension as well. When you are finished, the Slackware 13.0 will detect your 4GB of RAM. Look the results using free or cat /proc/meminfo