Because the T4 timer these is so easy to mount anywhere, they get put in the worst conditions you can think of, like directly over fuming chemical tanks. Even though the enclosure is rated IP65, the foamed gasket on our old timers is not designed to withstand chemical attack.  Also, there is a definite range that the screws can be tightened.  If torqued unevenly, the cover can warp and leak.   As a result, we have seem a small minority of our timers returned to us for leaking, and sometimes even with the gasket dissolved!  

Our new timer enclosure has a solid PVC elastomer gasket instead of the old foamed gasket.  In addition the cover wraps around the gasket to protect it from direct contact with chemicals.  The result is a timer more robust that the original T4.  We know from our experience with the T5 timers that have been using this type of enclosure for about a year that this will allow these timers to survive almost anything.

In addition, the substantially thicker plastic of the new enclosure prevents the lid from warping even when overtightened.  In fact there is not such thing as over-tight screws with the new enclosure.