2 routers and 4 peers route export selection

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