Tachometer not working

Hello,

It sounds like your battery is weak or slightly discharged. You mentioned that it's only 4 months old so it's most likely not a bad battery. You more than likely have an electrical draw somewhere in your vehicle that slowly drains it over a period of time.

I would start by performing this quick battery load test(see link below), just to make sure we don't overlook the possibility of a bad battery(sometimes they are defective). Any local automotive parts store will also test your battery free of charge if you're more comfortable with that.
https://www.2carpros.com/articles/car-battery-load-test

Assuming the battery passes the load test, the next thing you need to do is determine if there is a draw with the key off. The link below will walk you through this process.
https://www.2carpros.com/articles/car-battery-load-test

Let me know what you find!