pyln-spec: add testnet4 support

Author: ShahanaFarooqui

contrib/pyln-spec/bolt1/pyln/spec/bolt1/text.py

@@ -11,6 +11,7 @@
 - Bitcoin mainet with port number 9735 or the corresponding hexadecimal `0x2607`;
 - Bitcoin testnet with port number 19735 (`0x4D17`);
 - Bitcoin signet with port number 39735 (`0x9B37`).
+- Bitcoin testnet4 with port number 49735 (`0xC247`).
 
 The Unicode code point for LIGHTNING <sup>[1](#reference-1)</sup>, and the port convention try to follow the Bitcoin Core convention.