Need your opinion guys

[72Z/28]

I bought Nitto NT01 tires from summitracing in 2016. The production date of the tires I received were in 2015. They were almost a year old from the date of purchase.

I emailed Summitracing team regarding the tires production date, and they were almost one year old. They simply replied that they had sent me whatever was supplied by the manufacturer.

I kept the tires in the store, and good a chance to install them sometime in December, 2017. I have driven the car with the new tires for about 20-30 miles only.

The car has been sitting in a garage that is shaded from the top but exposed to sunlight from the sides ( outdoor garage sort of thing).

I removed the tires to get the wheel centers powder coated.
The rubber on the tires seems to be hard. However, have not noticed any cracks on sidewalls. Not sure if it is normal with those Nittos.

I just would like to get some valuable input guys whether I should keep the old tires since they are only 4 year old or be safe and get new tires.