This response is in case you are talking about the REAS package ($299) and not the Romance Plan ($599)
I can only speak from my personal experience.
Both last years
DCL Brochure and the current one, clearly state that the Romance Escape at Sea (REAS) can be added to the 3 day land, 4 day sea package. There is an asterisk next to the price. When you go down to the bottom of the page it says "Add-On prices above apply to the 3-night land / 4 night sea packages".
When I tried to add it to my Land/Sea through my
travel agent, they were told that I could not do so (they claimed it was a typo...two years in a row?). My TA went through a DCL rep and two supervisors with no luck. However, they then contacted their Disney rep. It took two weeks but they added it to my vacation and the famous Debbie made the reservations.
If they tell you they can't do it, fight.
I do not believe that you can add it to the 4 day land, 3 day sea.
Other board members have put forth the theory that the REAS is getting too popular and causing difficulties in accommodating reservations on the first afternoon of the cruise (otherwise known as the ressie scramble). That may be why they do not accept the REAS for the 4/3 package, because that would take too many reservations out of the smaller cruise's mix.
I can tell you that despite making the reservations 28 days out for a non-sold out cruise, I was unable to make spa reservations for the day at sea unless I wanted 8AM or 5PM.
It may be possible to purchase the REAS for the 3 day cruise if you book the land/sea separately.