Designing a bus - Master at one end or in the middle?
The location of the bus master doesn't matter.  The important point is whether the bus is a transmission line or not, and therefore what steps need to be taken to ensure signal integrity.  As Stevenvh pointed out, at 4MHz these are just digital signals on your board, and you probably need to do nothing special.

If the speed and or bus length went up such that it becomes a transmission line (lumped model no longer holds), then you have to deal with it as such.  There are various things to do, depending on how far into transmission line territory you are.  Common first steps are making sure the bus is only a single line with short drops (no star, for example) and terminating the ends.  Sometimes just adding low value resistors to each bus driver is good enough.  Diodes to clip overshoot and undershoot can help.  However, if all this is done right it still doesn't matter whether the master (presumably the bus driver?) is in the middle or one of the ends.