Dang, Papa! Way to come through...
4 days at Portofino - you are good to go, the hard work is already done!
Spend the first day getting acquainted - ride what you want, but get a feel for the layout of the hotel, City Walk, and the parks. Don't underestimate the shows, they are very fun. Plenty of good food, no need for reservations until you are on site, and probably not even then - maybe one or two of the restaurants at most.
We went early to the parks, then back to the hotel pool from 1pm to 4 pm, had dinner at City Walk and closed the parks. There seems to be a mad rush out of the parks between 5 and 6 pm, the evenings are a fantastic time to tour.
At most, look at the maps and the menu sticky - don't think you have to commando Universal like Disney. The Express Pass is golden and will allow for so much fun AND relaxation.
Pending your grandsons tastes - spend time at Harry Potter (both parks), the Mummy if tall enough (it is Great), Transformers/Spiderman, Minions. I also highly recommend taking an early shift and heading over in your swimwear and ride the 3 water rides at Islands (Jurassic Park, Dudley, and Blutos). They are so much fun, and the kids will get soaked - like stand in the shower with your clothes on wet!