This will depend somewhat upon the thickness of the sheet metal. A good fab shop can probably bend up to 1/2" thick or so, but a shop that does heating and A/C duct work might have trouble with even 16 gage (about 1/16" thick). Sheet metal has "spring back". So, if you are going to try to do it yourself, you will need a smaller diameter form than the 4 1/2" radius you need on the finished product. Just how much smaller depends on a number of factors. ---Takes experience. Since you are doing only one of these pieces, I'd suggest finding a shop to do it for you. Look up "welding" or "fabricating" in the yellow pages. Tell them what you want to do, including the thickness of the stock and the accuracy to which it has to be done.