Furnace third burner won't come on
I agree with the response above it is imperative that your gas pressure be right. Manifold pressure is 3.5" in our area. Although if you pressure was significantly down I doubt if any of the burners would light. Let me tell you what I ran into one time.  I basically had the same thing, three burners would light just fine but nothing on the fourth.  I checked my pressure, all good.  It was also the first start-up in the fall.  I pulled the manifold out and started looking at orifices.  I was checking the bore on the orifices.  I looked into the #4 orifice and I saw what looked like a white film.  Now I'm totally scratching my head.  I unscrew the #4 orifice.  Out comes a family of I mean tiny spiders.  I had never seen spiders so small, like half the size of a period on the end of a sentence.  They were so small that they could easily crawl through the orifice.  The white film was actually spider web.  They had webbed across the orifice opening from the inside.  The 3.5" of gas pressure was not enough to break through the web.  It totally blocked the gas.  I asked the customer for a Que-tip, swabbed out the opening, put it all back together and away she went.  Good Luck!