Which board are you trying it with? You'll have to change it in BOARDNAME.py - that auto-generates platform_config.h (which I guess you modified) and a bunch of other stuff, including the variable Serial4.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
Which board are you trying it with? You'll have to change it in BOARDNAME.py - that auto-generates platform_config.h (which I guess you modified) and a bunch of other stuff, including the variable Serial4.