First read this Ant tutorial. Then see my comments below...
1.Get update from svn for All a,b,c,d project.
9.Then I want to commit this ab.war archive to a particular svn location or tag it (for read-only).
You can try SVNAnt or just make external calls with <exec>
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2.clean all four project.
3.Build all four project.
5.Build b project.
7.Build a project.
4.make jar of project d and export into lib of project b.
6.make jar of project c and export into lib of project a.
8.Make ab.war archive of project a and b and put into specified location(i.e. c:\xyz)