I have done the 6 day version twice, and am going again next year in July.
I have not done the 4 day, so I can't directly compare it.
The highlights for me are:
Imagineering
Disney Studios
Jim Henson Studios
The special behind the scenes things you do at Disneyland/DCA (we have done backstage Soarin, backstage Indiana Jones, backstage Radiator Springs).
Being the only park guests in front of the castle in the morning.
The lowlights for me:
The Hollywood Day (you can do almost all of this by yourself pretty easily) (except for the Jim Henson tour)
The Batuu experience we had on the second trip was subpar (we were there within a couple weeks of it opening, and it wasn't that great, it's not like that anymore though).
The meh:
The "VIP" seating you get at any of the spectaculars is not anything special. In both trips I have been on, they just reserve part of the street, and have you sit on the ground (i.e. no VIP chairs). It's a good view though.
The bus ride from LA to Anaheim and LA Traffic, but that is mostly out of their control.
The sad part:
On the first trip, we were able to visit the 1901 club in Carthay Circle (like Club 33 in DL). Pretty sure they don't do that anymore.
If the 4 day is no longer including Imagineering, and you've never seen it, I would definitely recommend going with 6 day.