The area at the north end is indeed for a 4th satellite at some stage in the future. Gate utilisation is still not at the level to require this extra space, runway capacity at peak times and congestion at security are more significant items. The way Stansted has gone low cost with quick turnrounds means the ratio of gate numbers to runway capacity is way down on the design criteria of the 1980s. There aren't lots of long-haul widebodies sat at gates for hours like at Heathrow/Gatwick.
Access plans change over time. Satellite 1 has the cumbersome, very expensive and prone to breakdown Transit, one of those oh-so-brilliant design features of the terminal (like the unventilated toilets situated right in the middle of the main building concourses which pong everything around them) which got architect Sir Norman Foster all his design awards for the structure
; notice the Transit has not been repeated on Satellites 2 (Domestic) and 3 (Ryanair).
For low cost carriers look for a walking pier from the terminal. And when the Transit wears out, look for the same to be installed at Satellite 1.