Just playing with it a little.
Open Picasa and go to the folder of photos you want to use. Select 4 photos and in the menu, select create picture collage. It will open a new tab with the photos you selected in the collage area and all the photos from your folder in the area on the left. If you click the settings tab, you will be able to change the size to 4x6 and the grid spacing, etc. Then click create collage and it will move your picture into a collage folder with the 4 you chose.
Unfortunately, I don't know how to go around it to change photos without just creating a new collage, but hopefully, this will be enough to get you started.