I think expecting her to understand and/or keep promises at 4 almost 5 is unrealistic. Even when they are 3 times that age you can't always expect them to keep their promise. But even more so at your daughters age. She just found out that saying the word "promise" gets her what she wants, so that is what she's doing. She doesn't understand everything behind a promise. I really don't think promises should be stressed anyway. People are not perfect and people make mistakes. Enforcing and stressing about, promises, only means hurt feelings when they are broke. Instead rules and consequences should be put into place.
I think it sounds as though you just need to lay out the rules better.
Here is how I would handle your examples.
"1). We took her to the stock show and told her that since it would be crowded we couldn't bring the stroller and she'd have to walk a lot and she says "That's ok, I promise I won't complain" (she complained)"
For this one, it is inevitable. I mean she is 4, her little legs are bound to get tired. Maybe you could have suggested a rest period, where you sat and rested. Or you just learn she's not ready for this type of thing, yet.
"2). I bought her a hot dog for lunch and was going to split it with her and she said "No, I want the whole thing. I promise I'll eat it all." (she didn't)."
I would have replied, well lets order one and split it, if you are still hungry after that, we can get something else"
"3). With said hot dog above, she promised she'd eat it for dinner (she didn't)."
Since you went this route, provide the hot dog for dinner. If she chooses not to eat the hot dog, then she gets nothing else
"4). She wanted to wear her special barrettes for daycare, she promised she'd put them in her bag and not lose them (she lost one).
etc..."
I'm sure you won't intentionally loose your barrettes, however, when playing with a large group of kids they are bound to fall out and get lost. How about you pick out another pair and you can wear these ones....(pick another day).
5). She wants goldfish before dinner and promises she'll at least try everything (she doesn't).
You can't really expect after just eating goldfish that she is going to be hungry enough to try items. This is just a learning experience for you.