use 4 cores for routing

diff --git a/openlane/4ft4/config.tcl b/openlane/4ft4/config.tcl
index 9ba8c1d..3ab25bd 100755
--- a/openlane/4ft4/config.tcl
+++ b/openlane/4ft4/config.tcl

@@ -50,5 +50,8 @@
 set ::env(GND_NETS) [list {vssd1}]
 
 set ::env(DIODE_INSERTION_STRATEGY) 4
+
+set ::env(ROUTING_CORES) 4
+
 # If you're going to use multiple power domains, then disable cvc run.
 set ::env(RUN_CVC) 1