Can a problem with L4-L-5 disk cause pain at medial knee? Can it cause weakness in thighs? Thanks

Sometimes. L4/5 disc usually affects either the L4 and/or L5 nerves. If the L4 nerve is affected (in the foramen) then the thigh can be mildly weaker. If the L5 is affected, then the foot strength will be affected. Please see a doctor for a full exam.