discipline ?...daughter and stepdaughter hitting each other

[Guest guest]

My daughter is 4 years old and is speech impaired at a 2 year old

level. My stepdaughter is 6 and is not speech impaired. A recent

discipline problem has risen. The six year old can come up to me and

tell me if my daughter hit her. But, she won't come up to me to tell me

if she hit my daughter obviously, and my daughter cannot tell me. How

do I handle this situation? I have no clue where to start! Thanks