def METHOD_NAME():
    mem_stores = [MemoryStore(str(i)) for i in range(4)]
    store = ConcatStore(mem_stores)
    store.connect()
    index = 0
    props = {i: str(i) for i in range(10)}
    for mem_store in mem_stores:
        docs = [{"task_id": i, "prop": props[i - index], "index": index} for i in range(index, index + 10)]
        index = index + 10
        mem_store.update(docs)
    return store