Just weigh out an eggcup full of pellets in Grams, multiply by 3 to find out how much 3 eat in a day, then divide 4000 into that number
For example. Say the amount is 50g. 50 x 3 = 150g
4000 / divided by 150 = 26.6 days
The amount in the eggcup may vary slightly but it will give you a pretty good guide as to how long a bag will last.
I use this method to work out how long a bag of food will last our dog before we got him and I was trying to decide what to feed him on