I have several 5 gal buckets of grain - un-crushed. I do not have an accurate scale (yet).
Roughly - how much does 4 gallons of Maris weight?
I was just able to fit 50 pounds of uncrushed two row into 2 five gallon homer buckets. Therefore each gallon holds about 5 pounds. So, with 4 gallons you should have about 20 pounds of Marris Otter, give or take some. Hope that helps.
I have found grain to be a bit less than 1 qt per pound. I would guess 22 pounds.
Digital Scales at your LHBS or even Bed/Bath and Beyond type stores are fairly inexpensive. Run out "TODAY" and pick one up. You need it!