Once you update your Expression web 4 software with Service Pack 2, you will find making rounded corners has never been easier.

CSS3 Update:  We no longer need to use Vendor Specific Prefixes with the border-radius property.

For instance, -moz, is Mozilla's prefix that we use to target Firefox.  So in the past, to round corners in Firefox (and other browsers), we had to write styles such as the following:

Today we can eliminate -moz and -webkit prefixes and simply use border-radius.

For rounding corners in Internet Explorer versions 8 and earlier, See PIE.

Expression Web 4 Update:  "Border Radius" Category in Style Builder

Notice in the above screenshot, a new category was added to the New Style Dialog window........ Border Radius.

We no longer need to write our own CSS3 Style declarations just to round the corners of page elements!

Expression Web 4 Update:  Box-Shadow Property added to Style Builder