how can i fix this error: serial8250: too much work for irq4 EC2 Instance

I think there are several things here.

First, the serial8250 message: This means that there is some process (or multiple processes) writing data to the instance serial port. This is normally used to output debug messages (see EC2 Serial Console for more information) - so check to see what logging is enabled and what on your instance is send things to the serial port.

Normally this won't affect anything else - it's just a warning to say that there is too much data being sent for the serial port to keep up - they are generally low speed devices when compared to Ethernet. In the dim, dark, historical days of computing it was possible for a single interrupt to consume too many CPU resources and therefore cause performance problems but that's very unlikely in the modern world.

The "Service Unavailable" message you're seeing is because there is some error in the web server software on the instance. That you're getting a 503 error means that the web server is running fun; but whatever it is trying to do is throwing an error. I doubt this is related to the serial console error message. In this case, look at the web server logs to determine where the error is.