Personally, I think a memory box would be just the thing. Something really nice like one of those Shadow Box frames (they sell them at Hobby Lobby). It's not terribly expensive. Put in a collar, maybe a few pictures of her and Casey. I think it would also be very helpful (especially at this age) for her to maybe stop, and take the time to write Casey a letter about how she feels, what she misses about her, what her family is like now without Casey and then put it in an envelope, and place it in the box too. Don't totally freak out if you can't find the collar. Ask your daughter (if you can't find it) if she'd like to go pick out a special collar from the pet store (maybe even a nice name tag too) so that she can put it in the box on display.
As adults, we display grief in so many different ways. Some of us drink, we get mad, some act out in anger, some blow off death (like if we ignore it, it never happened), some mourn for years, even falling into deep depressions (that's why all those pharmaceutical companies are getting rich!!). Then there are others who become obsessed with it. Is death coming after them! We go on diets, buy new exercise equipment we will never loose. We go on a diet, go to the doctor (over EVERY little thing we feel) because we don't want to go now either. Adults are FREAKS about death. We would like to think we have got it under control but in reality, we don't. So, it is totally acceptable and rational for your daughter to get a little wiggy with all of this. Please don't take this the wrong way, I am only guessing here, but when Casey died, your daughter was only 3 or 4. There is a good chance that no one really talked to her about death and that things die and helped her get through it (over a long period). Talking about it at the time probably did help, but as she gets older and her sense of self and community grows, she becomes more aware that things go missing! For a Child, death is not just a thing, it is a whole process they go through which could last for years because they don't have the skills or maturity to deal with it properly.
I think that as her mom, it is important to let her know that her feelings are important to you. Acknowledge that she is grieving and HELP her get through it. It's not enough to say, "yeah, I know you miss her but the dog died 4 years ago, get over it". Sit down with her, show her pictures you have of the two of them together, talk about Casey and the things that even might have led to her death. Then, if you decide to do the box, you can call it her closure box. Let her know that you will never forget Casey and you don't want her too, but that Casey wouldn't want her to go on crying and be sad forever either.
My son lost his auntie almost 2 years ago and he is now 6. He goes through the same thing, tears and all. some days are worse than others, but the one thing that helps him daily is a hanging Angel plaque that auntie gave it. It's in his room for him to look at every night. but he does bring her up often and we just talk about the wonderful things we did, but also how sick she was and that she is not suffering anymore. It helps.
