you have a sqr stage 4 tune but no IC to go with it?!?!?
why would you go through putting in a bigger turbo with no IC?]
you want the coldest air you can get in your engine, so on a turbo car the air goes throgh the turbo and gets compressed whick makes it hotter than when it came into the intake. a intercooler makes the air cool down by having th hot air go through it as cooler air passes over the intercooler cooling it down. so heat soak is when the intercooler is to small or poorly made or has not enough air flow and does not cool enough, this will make the air going into the engine hotter which makes the exhaust hotter which makes the turbo hotter which makes the air going into the intercooler hotter and it just goes around and around.
look around on ebay for some intercoolers, you need to make most of them fit but it might save you some cash over a forge or mp one.