After mastering such skills as balance, ramps and offroad riding (among other things) it's not time to hang up the Strider just yet. Upgrading to the extra long seat post will make it useful until they are 5!
This is a great way for them to continue working on their balance skill without being hindered by pedals. It also gives them an opportunity to be a little more daring. Whether they start getting some air off of curbs and ramps or they get more comfortable "shredding the gnar" (We're not quite sure what that means, but probably not this) at their local skate park.
A lot of Strider riders will continue to go back to their pedal-less friends even after starting to ride a pedal bike. Remember it's all about fun and honing their craft!
Here's a great video showing a lot of the skills that your rider will continue to develop.