Could a 4 yr old born to a mother who was using meth during the pregnancy have learning difficulties? If so, what kind?

Yes. It is really unclear as to the precise nature of developmental problems in a child exposed to methamphetamine in utero. Cognitive, emotional and physical development can certainly be affected. Such a child is a risk for problems later in life. It's important to do the best for the child moving forward, including, but not limited to parenting classes to learn best to treat the child appropriately.
Yes. Generally, i would expect that if learning disorders were present, they would be those that are commonly associated with adhd. In the womb, the presence of stimulants may tell the fetal brain it doesn't have to produce its own. After birth, when it no longer has that exposure, it may struggle to produce its own natural brain stimulants as it has become dependent on those from mother.