I think these are very nice for smaller turtles. I, like
you, keep my turtles out in a protected pond in the summer
and bring them in to a 120 gallon aquaruim for winter.

I don't think they are very good for large turtles like
the RES and painted turtles who enjoy deeper water(with
a basking area of course). My RES is 8 years old and
about 3 lbs.

For one 4 year old turtle you could probably get
a few years use out of it.

Hope this helps
Linda