As a fourth grade teacher let me hopefully shed some light on what seems to be two different issues here:
How the teacher of your child advised you, and
Whether or not advancing your child's fraction knowledge is relevant or necessary.
Addressing the first question, if this teacher did, in fact, say "don't get involved" without further explanation, then this teacher needs to learn a bit of tack. Let me try and rephrase what may have been meant; often times I suggest parents take a back seat to homework because of two things: if the assignment comes back complete and correct, but the parent had to walk their child through the entire process, then I don't get an accurate reading of the child's understanding of the topic. If I don't have an accurate reading then I can't know to give them extra attention, because I assume they understand it. Assistance on homework is always great, but I prefer parents to ask their child questions about how they learned it in school and to try and explain the process as much as possible before the parent jumps in. If the child explains something incorrectly you know exactly where you can straighten them out without having to entirely reteach the method.
The second reason we say, "don't worry about the homework" is because we respect your home life and don't want you to have the added stress of battling over a dumb work sheet at night, when everyone is tired. We are trying to be on your side.
The second issue is simple: If your child wants to learn the subject, they will. If they have the desire they can ask you questions and initiate their own inquiries without being prompted. This is how leadership is built. If you think they need a push, perhaps talk to the teacher and get his/her advice before raising your expectations. Failure is part of learning, but pushing them too much and setting them up to fail can be just as damaging as not pushing them enough. Multiplying fractions with different denominators isn't taught in our school until 5th grade, unless the child is clearly an accelerated math thinker.
I hope this helps