My TSH was 4.1 & my T4 was 13. Do you think I have a borderline underactive thyroid?

Pretty normal. aware that TSH is high normal at 4.1 but T4 normal. If concerned about hypothyroid, check TPO antibodies and check levels of TSH, T4 and T3 (liothyronine) every 6 months. Keep in mind the symptoms of hypothyroid very common.
Possible. What are your symptoms. That is probably the most important question. I recommend looking into a book called Hypothyroidism: The unsuspected illness by Dr Broda Barnes. Good lay source about Thyroid problems.