747-400 Krueger Flaps
 

Can some one tell me why they are only used inboard of engines 2-3 and not across the whole wing. (they use slats)

It is a design decision coupled with slotted slats outboard so that wingtip does not stall first. (main problem on sweep wing aircraft)

The whole span of every 747 has krueger flaps... the ones inboard of engines 2 and 3 just look different as they don't "curve" as much on extension like the rest. By curving I mean that the extension mechanism increases the camber of the main surface of the flap to increase its effectiveness.

But they are not slats by any definition.

The non kreuger flaps are variable camber leading edge flaps.