doc: use testnet4 in developer docs

Author: Sjors

doc/developer-notes.md

@@ -410,7 +410,7 @@ to see it.
 ### Signet, testnet, and regtest modes
 
 If you are testing multi-machine code that needs to operate across the internet,
-you can run with either the `-signet` or the `-testnet` config option to test
+you can run with either the `-signet` or the `-testnet4` config option to test
 with "play bitcoins" on a test network.
 
 If you are testing something that can run on one machine, run with the