Maybe it hasn't changed at all... but you (and he) have... But now after 4 years... you need more, or a shake-up, something introduced into the mix that rekindles the initial rush. All relationships go through these times where the fire smolders and something is required to stir the coals.

Have you thought about maybe... "resisting" him... make him work for your submission... which will make him stretch his dom-muscles to retame you... and will make you feel "newly" submissive as he does so.

Have you thought about exploring some soft (or hard) limits you left untouched the first time around.

Have you thought about involving a third party, if not in r/l... perhaps a "guest-director"...