What parts could be labeled XT and be near a Clock of a microcontroller
That is a ceramic resonator.  It is electrically similar to a quartz crystal, although less accurate.  The advantages of resonators over crystals are that they are cheaper, more mechanically robust, and usually require lower drive power.  The 3-pin variety comes with the load caps built in.

The "4.00" may mean 4 MHz.  The "J" is probably a accuracy or temperature or package code the manufacturer uses in the part number.  Check if Murata has something similar.