A t4 test is done to check for a condition called Hyperthryoidism. It's part of the senior panel at my vet's clinic.
A low measurement would be called Hypothyroidism and you may want to have that test rerun in a month or so and if it continues to show Hypothyroidism, then you should discuss meds with your vet. Hypert is more common and that's the one I'm more familiar with.
I think Hypot would lead to constipation, fatigue, weight gain. Basically the thyroid gland is not producing enough of the hormone. The thyroid gland controls a cat's metabolism, the speed at which the body converts food into energy (I think, hopefully others will correct me if I'm wrong). I think hypot means that your cat can't convert food into energy as well.
Any other signs? Weight gain, poor coat, itchy or flaky coat, fatigue? The condition can be easily treated with meds.