Peppermint 3 specific software (ie. the few packages from the PM3 repo) will no longer receive updates .. but by far the majority of software comes from the Ubuntu repos, and PM3 being based on an LTS means you'll get updates for the vast majority of your stuff for ages yet.
Beyond the fact that PM4 is slicker and has more features such as a built in compositor, transparency, Win7 style window snapping, etc. .. the 13.04 repo's are likely to contain updated versions of a lot of software that may never be added/updated in the 12.04 repo's
If you're happy with PM3, there is absolutely no "need" to move to PM4 yet .. do so at your own pace, or indeed don't and wait for PM5 .. the choice is yours