Sorry, I'm not going to work up an example of four pages using Struts which stores data in the session. That's way too much to ask.
And what detail do you want? "Store the information in the session" should be all the detail you need. You get the request from page 1, you extract the data from it and store that in the session. Same for page 2 and page 3. When you get the request from page 4, you extract the data from it and combine it with the data you stored in the session. Update the database with that.