The 4.0 changeset was:
ChangeSet@1.1579.3.1, 2003-09-26 23:43:22+02:00, guilhem@mysql.com
  Fix for 64-bit machines.
  I am almost sure this is the cause for
  BUG#1381 [Opn]: Bug in replication on HP-UX 64 bit binaries?
  BUG#1256 [CRp]: Replication slave fails to connect to master in 64-bit version
 (Solaris)
  The reason why I think it's wrong is that the normal client code has
  uint32 ip_addr.
  (of course on 32-bit machines it does not matter, but on 64-bit it does).