You may benefit from rider switching.

If there are rides your 4 year old is not tall enough to ride, then you can basically get a free lightning lane for that ride. You or your wife will wait in the regular line (or LL if you pay for it) with your 6 year old, while the other watches the 4 year old. Then when you're done with the ride, the other parent can do LL with the 6 year old, so your 6 year old gets to go twice and is the big winner. We did this with Rock N Roller coaster. I took my 5 year old to the Cars show, while my wife took my 7 year old on Rock N Roller coaster. Then I got to do Rock N Roller Coaster with the 7 year old using LL.

Click here for link at https://disneyworld.disney.go.com/guest-services/rider-switc