Converting latitude, longitude points into a polygon.

I have a data having four latitude and four longitude points. I have to create a polygon shape file from the points and divide the polygon into grids and make many small polygons. Please help me.
Here is the sample:

Latitude Longitude
61.0000 -145.0000
59.3000 -145.0000
54.7000 -156.0000
55.8000 -157.5000
61.0000 -145.0000