In all the times Ive had a colonoscopy Ive never finished the prep in 4 hours. Seriously though, drinking it through a straw helps a lot. Usually I can get through half of it fine, but the second half kills me! I just hold my nose and suck it through the straw fast. I fill up a glass and try to get 1/3 of it down at a time, every 5 mins or so. Also, I know you said you suffer from cyclical vomiting, I dont know what that all involves or if it has to do with your gag reflexes, but I put the straw deep in my mouth so when I suck in it just goes right down my throat and I hardly taste it. Also making sure its SUPER cold helps a lot too. Goodluck! I hope you can get it down.
EDIT: Also, I never start having BMs until Im almost completely done with it, a lot of the time I worry that its not working, but it does once you start getting the second half done.
Dianogsed with Crohns: At 16 years old. 22 years old now.
Surgeries:2 Bowel Resections, Gallbladder Removed.
Current Meds: Imuran 50mg, Vitamin B12 (injection), D, and C tablets. Fish Oil Tablets, Cats Claw, Slippery Elm, and Reishi.