40000^pixels? a heightmap? thats crazy
if theres gaps between your models then your model must be off sligtly, and you could just adjust the positions of each part separately, if you truely want a huge landscape you could either use advanced terrains and use a big heightmap, or join some big advanced terrains together or use a matrix but shift it every so many units you walk across and change the heights of the points thus giving the illusion of a large lanscape, or use a 3d model alndscape and possibly slice it up a few times.