My tsh was 4.1 & my t4 was 13. do you think i have a borderline underactive thyroid?

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.

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.