How big will my dog be when full grown? We have a 4 month old Maltipoo who now weighs 5lb 4oz

A maltipoo is a mixed-breed dog bred from Poodle and Maltese parents.While maltese dogs tend to have a generally uniform conformation, poodles can vary widely in size.
So there's not an exact height, though it would most likely be 3 ft. and under.

There was a discussion of this on a dog forum I am active on. No 2 of us seemed to have had the same experience. One consistent pattern seemed to be, the bigger the dog, the longer it keeps growing. Four months is about when I look at my Labs and conclude I have been stuck with another runt. At that point, I think they have about 80% of their adult height. So I doubt your puppy will ever be much bigger than it is.

Did you look at at the sticky starting at https://www.askmehelpdesk.com/dogs/information-articles-our-dogs-expert-labman-53153.html#post251802

3 ft and under? Dogs usually are measured at the shoulder, and not even a standard Poodle reaches 3 feet. Likely Your Maltipoo came from a toy Poodle.