Can i change a 5 letter pass code for an iPad to a 4 number pass code?

The following document describes how to do this:

Use a passcode with your iPhone, iPad, or iPod touch - Apple Support

However, please understand that depending on the model of iPad hardware, and settings you have chosen, the data on your device will then be far less secure than it was. You should really be using at least a 6-digit passcode on all of your devices.