Hi!
If in a week you could not find the answer, then try this option.
First of all, set fixed addresses for each of the 4 asterisk.
Next, regexroute.conf:
[once]
$ip_site1=192.168.110.10
$ip_site2=192.168.120.10
$ip_site3=192.168.130.10
$ip_site4=192.168.140.10
[default]
${rtp_forward}possible=;rtp_forward=yes
^4=sip/sip:\0@$(ip_site1)
^5=sip/sip:\0@$(ip_site2)
^6=sip/sip:\0@$(ip_site3)
^7=sip/sip:\0@$(ip_site4)