Getting Stamp4 with Greencard

Hi,

I got Greencard visa(processed by my employer) in May'2013. But, because of delays in project proposals, my travel to Ireland was delayed.
I landed in Ireland in February 2014. And paying tax from February'2014.
My greencard is valid till May' 2015 and it is my 1st Greendcard visa to Ireland.

Now, I have below queries -
1) If I apply for extension of my Greencard, will I get Stamp4 ?
2)Can I change my employer now ? If yes what should I require ? If no, what are the reasons ?

Could you please clarify my queries ?

Thanks in advance.

Murali