I know that $4k is a lot of money, no matter how you slice it, but just how big is the group you are working with?
I use US Food Service for our accounts(so I am not in Canada), but honestly, we have 11 accounts, and the average invoice(and 3 deliveries a week on average)all average around $4K each, you can do the math, and even then, or sales rep can be a little shifty/hard to get hold off. . .BUT they are ALWAYS pushing specials, ALWAYS willing to battle pricing, and never ONCE have I said to them, "well, I see that company X is offering this same product at $.16 a can cheaper, and I am buying 20 cans per unit, across 11 units, so that adds up", and they ALWAYS come back with at lest SOME sort of compromise, or price change. It shouldn't matter the size of the operation though, they should be there to help you, and even more so because you are a smaller operation, and they should set a good example by helping the local business guy, trying to get by and make a buck too. . .that's what is is all about, isn't it?
Things, specifically with produce, sure there are market fluctuations, and sure they can buy a little further out, but I think it's BS the line they are feeding you. If they are paying x amount, and can see that the other places are getting it far cheaper, than that is a bad business model on their part, and YOU are suffering.
Do you have a local produce company that you could open an account with? They tend to be a LOT more dynamic in pricing, and the product is always more consistent as that is ALL they deal with(and occasionally dairy). Is there anything you can do to call her bluff, I mean, if you got it to where you only got stuff from her that was based on quarterly pricing, and only things that hit your dry storage/freezer, and janitorial compartments it might show her you mean business, and you will see just how much that $4K a month to them really means. . . not that it should matter, they are in the CUSTOMER SERVICE BUSINESS. . .but they HAVE to be able to do more to work with you, it's just silly that they would run a business like that.