I'm not an expert in O/A welding (self-taught, so take this in stride), but I do believe that a #4 is too small to do any serious welding of 1/2" steel. Now, if you're trying to weld a small piece of 1/2" it might work (like welding a 2" square vs a table-top), but it'll take a long time to do so.
I don't have my charts in front of me, but I did find this page
which shows a comparison of different tip sizes and steel thicknesses, and you're looking at a size 6 Victor tip for 1/2". This would also put you into a medium-duty torch body at a minimum (I use a Victor 100-series).
Welding 1/2" with an O/A torch is do-able, it might take a couple passes to get full penetration depending on what you're doing.