not sure how big the pond is, but there are 325,000 gallons in an acre foot. So, the 4 inch pump would fill a 1 acre pond, 1 foot deep in 21 hours. If the ponds depth is 7 feet, you would basically have the pump going 24 hours a day (assuming a few hours a day of downtime when it runs out of fuel) for a full week to fill a 1 acre pond that is 7 feet deep.
Most folks don't understand how much water it takes to fill a "pond". Small water pumps 3 inch and smaller are really impractical in filling ponds, unless the pond is very small. I hear quite often, I will run my pump to fill the lake, the reality is most small pumps can't even keep pace with evaporation in the summer time.
The other option is to spend a lot more money and get a 6 inch pump. Now your in business.