Replacing 4 instead of 5 tires on a used trailer?

I am planning to replace my 4 trailer tires at the end of this season, and only because of age. They are 10 years old. They have great tread, no cracks, stored off the ground during winter, and covered for 9 months of the year. I will not replace the spare, it has not been off the back in 10 years.