Re fractional inches, any machinist or welder who memorizes these will end up saving a lot of time:
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For sixteenths, a good memory trick is to just memorize .0625, .1875, .3125 and .4375; then just add .5000 to get the 5/16 (.5625), 7/16 (.6875), 9/16 (.8125) and 11/16 (.9375).
Carpenters love a tape measure and working to 1/16” or 1/32”.
For your 4” round bar, your local steel supply should offer a cutting service. It may be a hassle to take the one piece down there, but not so bad bringing four pieces home. My LSS would charge about $9 for each cut of 4” round stock.