Pretty sure that Bethesda has explicably stated numerous times that they are done with Oblivion, and will NEVER touch on it again as a game. I've seen this post from Gstaff numerous times myself a few years ago.
If Bethesda didn't say it, don't believe it.
okay, here:
http://www.google.com/#q=oblivion+fifth+game&hl=en&prmd=ivnsu&source=univ&tbm=nws&tbo=u&sa=X&ei=C6LeTd3tCOzPiALrv-HNCg&sqi=2&ved=0CDIQqAI&bav=on.2,or.r_gc.r_pw.&fp=b8ca67d52bcf69c4&biw=1259&bih=823&safe=active
Look for yourself.
Still not convinced?
here again:
http://www.411mania.com/games/news/187189/Oblivion-Getting-Fifth-Anniversary-Edition.htm
http://www.google.com/#q=oblivion+fifth+game&hl=en&prmd=ivnsu&safe=active&um=1&ie=UTF-8&tbo=u&tbm=shop&source=og&sa=N&tab=nf&bav=on.2,or.r_gc.r_pw.&fp=529f0a0ec192b0c7&biw=1259&bih=823