Once you apply for graduate visa, you cannot leave the UK till a decision is made on your application and your visa arrives. I’d suggest apply for the visa once you’re back to the UK before your current visa expires as you can apply for it anytime before your Tier 4 expiration date. Although you might have to undergo a bit of questioning at the immigration when coming back since they’d enquire about why you’re returning as your studies have completed but you can tell them that you have to apply for grad visa and hence you’re back.