Spidey 5 and 6 were up until now precautionary rumours but now the balls is officially rolling as sony have hired James Vanderbilt to write both films as back to back sequels to Sam Raimi's 4th film.
The Spiderman franchise is a serious money maker and they are keen to get 4 out as soon as they can with the 5th and 6th following soon after.
Kirsten Dunst, Tobey Maguire and Sam Raimi are not signed on for any more films and it is unlikely that they will return after 4. Many rumours are being kicked around about an entirely new cast for 5 and 6, which are to tie together plotwise.