Is the 4-hour river for tubing open year round?

Yes

You need to check the website

You can use the river all year around. But tube rentals are seasonal

If you bring your own gear it is. The rental shop sometimes closes.

We were there in February and were able to take our kayaks down the river.

No it starts April 1sti was just there and it was not open and they told me it didnt open until April 1st

Yes it is