Junie B. Jones books are short and funny. Very similar to Ramona. Also, my daughter enjoys the Lemony Snicket series and the Treehouse mysteries. The Boxcar children is a good series too but they are a little bit longer.
What age is third or fourth grade?
The "House a Pooh Corner" is actually a group of short stories....Any of the Pooh books are usually enjoyed...and they fun to read with different voice characteristics. Pooh with a low, Piglet high and Eeyore nasal....etc. She could help you read some of it.
The last three all contain the same characters...the cat is convinced that the pet bunny is a vampire sucking the juices out of vegetables...heheh. Very harmless and funny.
I am trying to think of some more. It has been a long time since my children were in that age group, and my grandson is reading higher level books now. I will send you a note if I think of any additional.
The Little House on the Prairie books. I read them all to my son (who is a college senior now)when he was little, and we still refer to things that happened in them. You'll enjoy them as much as she will.
Not sure if you'll be interesting in these books .. but they are a series call ..
They are diaries from certain time periods in history
I picked up a few for my middle daughter who in 4th grade and who hates reading .. they are an easy read
My sons enjoyed Rudyard Kipling at that age. [Rikki Tikki Tavi] and others.
We read the Chronicles of Narnia and really enjoyed them.
I loved the boxcar books when I was that age.
But I have a passion for nonfiction, and when I was in the third grade started reading juvenile biographies. My sons also loved to read science type books, nonfiction books about animals. Almost anything would hold their attention. One loved reptiles and the other loved sea creatures [whales, sharks, dolphins and not in that order].
I'm not a big fan of fiction works in general, especially modern subject fiction. They just are not that well written. Didn't hold my sons' attention.
Might try Dickens' Christmas Carol later in the year. He is my favorite author bar none, and that particular story is fascinating even for younger readers.
A book that is challenging for your mentee, that you might have to help her with some of the words, might push her along to bigger and better things. She might love reading more if there were big ideas in the books, something important, something to make her think. Not just reading to be reading.