For the 73 result there must be something more than just units. Please double check the lab report. If that is what it says, then I would check with the doctor's office. When I look up TPO ab reference range, the first 3 sources I located listed the range as 0 - 35 IU/mL Where does the <4 IU/mL come from?
Even more important than lab test results are symptoms. What symptoms do you have, if any. Also, if other thyroid tests done, please list those, along with their reference ranges shown on the lab report.
Are you sure the books you are reading are talking about TPO ab, not TG ab, which does have a range like < 4?
At any rate, your TPO ab test result is above either of the 3 different ranges, indicating inflammation which may be due to an autoimmune condition: Hashimoto's Thyroiditis. This is not consistent with your TSH level, so you may be in the early stages. More importantly you listed 3 symptoms that are frequently related to hypothyroidism. Along with that Your Free T4 of .8 is actually below range and your Free T3 of 2.8 is only at 33% of its range. Those are lower than needed by most people. The Free T3 being higher in range than the Free T4 may also be an indication that your body is try to maintain thyroid function as best possible by converting more T4 to T3. TSH is affected by so many things that a TSH result within range does not preclude hypothyroidism.
Since hypothyroidism is not just inadequate thyroid hormone, but instead is correctly defined as insufficient T3 effect in tissue throughout the body, as a result of inadequate supply of, or response to, thyroid hormone, there are also other factors that have an effect. You have already tested one of those: Reverse T3 and it was mid-range. At first opportunity you also should get the doctor to test for cortisol, Vitamin D, B12 and ferritin.
A good thyroid doctor will diagnose a potential hypothyroid patient by first evaluation for symptoms that occur more frequently with hypothyroidism than not. Following that all the tests you have had and that I mentioned need to be done to help support clinical evaluation. If the diagnosis is hypothyroidism, then the patient should be treated with thyroid medication adequate to get FT4 to mid-range, and FT3 into the upper third of its range, or as needed to relieve symptoms, without being influenced by resultant TSH levels. Symptom relief should be all important, not just test results, and especially not TSH results when being treated with thyroid med. In addition of course the other mentioned tests must be confirmed as optimal, or supplemented as needed. If your doctor is unwilling to diagnose and treat clinically, as described then you will need t find a good thyroid doctor that will do so.
I've had TPOAb tested at both labs here in Queensland Australia. One lab states normal is < 60 U and the other lab < 6 IU. There is no converting between the two methods so I was told by the labs but interestingly my result at the time was 460 U and 46 IU. I have had TPOAb up to 1900 U but never had an IU test to check the results. Selenium may lower TPOAb. Try one or two Brazil nuts daily and see what your TPOAb results are in 3 to 6 months.