Are you able to ping other interfaces? Does any of these work:
(R4)
ping 172.27.0.17 source 172.27.0.18 rapid
ping 172.27.0.22 source 172.27.0.21 rapid
ping 172.27.0.10 source 172.27.0.9 rapid
(R2)
ping 172.27.0.1 source 172.27.0.2 rapid
I haven't used Junosphere before, but are you able to capture traffic on the link between R2 and R4?