Do not accept routes from RIP neighbor 192.0.2.3, accept routes from neighbor 192.0.2.4 as is, and for all other routes increment the metric by 2:
set routemap rip-in id 10 on
set routemap rip-in id 10 restrict
set routemap rip-in id 10 match neighbor 192.0.2.3
set routemap rip-in id 15 on
set routemap rip-in id 15 match neighbor 192.0.2.4
set routemap rip-in id 20 on
set routemap rip-in id 20 action metric add 2
set rip import-routemap rip-in preference 1 on