From what I understand, there is no generic form of adequan for equine use.
I use Adequan on my horse, I did the first months does like you are doing and then after that I am able to do 4 shots at the 4 day interval every 3 months with great results.
Some people do the doses every 6 months, some do them monthly, etc. Depends on what works for your horse.
I was doing them monthly1 (meaning 1 shot per month) but the results weren't noticeable at that level.
"Be a best friend, tell the truth, and overuse I love you
Go to work, do your best, don't outsmart your common sense
Never let your prayin knees get lazy
And love like crazy"