Here are the manuals:
http://www.pentairpool.com/pdfs/sanddollarDS.pdf
http://www.pentairpool.com/pdfs/sandfiltergenericOM.pdf

You can empty it -- be CAREFUL of the laterals!! -- and fill it with 150lbs, or your can add sand till it's within about 6" of the low part of the water inlet on the bottom of the valve. If the existing sand is gummy or in big lumps, replace it.

By the way, you listed your maximum depth as 4' and your shallow depth as 4.5'. Not sure what you mean by that, BUT the maximum volume for a 15' x 30' oval with an average WATER depth of 3.9' is only 7,700 gallons.