they're your diagnostic LEDs. they're used if there is a poblem with the system.  Four green after the system is started up indicates a successful POST... For example if it was to stay at 2 amber and 2 green LEDs that would point towards a memory problem

Thanks, is there any way to check if my USB ports are working properly?

not using those LEDs. You could test the controllers in the diagnostics but other than that you should just try known good devices with the system or try a hard drive with a good build on it in the system with the hardware you're trying to get working. this should let you eliminate software without an OS reinstall