2 routers and 4 peers route export selection

[Laurent GUERBY]

Hi,

I have two debian BIRD routers R1 and R2 connected by an iBGP session,
both R1 and R2 are connected to two eBGP peers each:

P1 <==> R1 <===> R2 <===> P3
P2 <==> |        |  <===> P4

Some routes are imported by R1 from both P1 and P2 and I want to
continue to export to P3 a mix of routes from P1 and P2 (which is what
is done by the iBGP session and preferences) but now I want to export to
P4 only the routes imported from P2 and no route imported from P1.

How would you do that with BIRD?

Thanks in advance,

Laurent