Ever since big updates yesterday, Slackware has officially support ext4 filesystem for the next release. Ext4 was introduced in Linux kernel 2.6.28 and the kernel stock at -Current has already used 22.214.171.124 along with the initrd.img and usbboot.img which are the installer images, meaning that ext4 support is ready at installation time, according to AlienBOB's blog.
We have seen numerous improvements on ext4 which is not available on ext3, so i think it's a good step for Slackware to support this filesystem. You can also try to use ext4 on top of your ext3 filesystem, so the changes are not permanent (only when mounted as ext4). This can help you decide whether you would like to upgrade or not. Here are some of the main features in ext4: bigger filesystem/file sizes, extents, delayed allocation, multiblock allocation, improved block allocation algoritms, faster fsck, online defragmentation and faster and more robust journaling.