This should be in the P&M forum
I'm not familiar with
GK, but what you are asking is a little confusing. In your first post, you ask for something with a 4+ armor save. This is carapace armor, found on scouts and some
IG elites (stormtroopers, Karskin?) In your second post, you ask for flack armor, which is the lame 5+ stuff issued to
IG line troops.
4+ with a bolter is easy, get a box of
SM scouts
http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat440273a&prodId=prod1060077
5+ with a bolter is a lot harder.
IG troops would make for a good base, but you will need guns from elsewhere. While the sternguard box is an excellent source for bits, if all you want is bolters/stormbolters you could probably get them cheep via bits sites or trading with friends. Most marine players who have been playing for any length of time should have a handful of bolters in their bitzbox. Stormbolters are going to be harder to come by, but you can get them in the sternguard kit and the commander kit from
SM. As they are not a very good option to equip either of those guys, they often go straight from the sprue to the box.