Yes there are four polypeptide chains, but those can be divided in two light/short and two heavy/large ones. - Just for understanding. :)
You almost got it. The domain with the 5-stranded ß-sheets changes depending on the particular antigen, that shall be expressed. Nonetheless they are not randomly changed, but at the sites of the hypervariability regions, of which 3 can be found per 5-stranded ß-sheets. As two of those sheets associate to form one barrel, you will have 6 of those hypervariable loops.
I hope I could help. :)