I can't answer you about the relative value of an after-2 or after-4 Conference Ticket. I would only guess that, if it's less than a single day regular ticket, then it's a value. When I've attended conventions at the Dolphin, it was mid-to-late-afternoon before we were done. It would have felt like a waste to use a full-day ticket for what remained of the day. Sometimes we were going into Epcot just for dinner and Illuminations.
If Epcot or DHS have extra magic hours (EMH) and you want to stroll or boat over there from the Dolphin, your admission to EMH is based on what hotel you are staying at, not your ticket. All Disney Resorts, Swan and Dolphin, and Hilton Lake Buena Vista are entitled to EMH entry. Show your Dolphin room key to get admitted to the parks during EMH, and also at the ride entrances, where they check for resort ID.