Cortana (or what appears to be) messaged chief directly. Chief basically said he's going to Meridian alone. Blue Team are total bros and are tagging along because as Fred puts it "They can't court marial all of us."
Mission 3 has Osiris going down to Meridian via orbiter space elevator.
Vale gets her neck broken by Kelly's thighs.
They look so fucking sexy, I love the contrast
>out of nowhere
At the end of halo 4, chief is implied to finally get to relax and take off his armor for a bit and then do random bullshit.
It's perfectly logical that blue team was just chilling and they meet up again. There is nothing important to explain because nothing was happening at the end of 4.