When you have 2 Sup2+'s in your 4k, the first uplink port on each is active. There is no concept of active/standby ports here, so you're depending on spanning-tree to provide redundancy. When you remove the 1st SUP, we lose our STP root, which forces the other link to go through the STP states.
It is up to you to make this process quicker at L2. If both uplinks connect to the same switch, you can put them both in an etherchannel so there will be no loss when a SUP/link is pulled. If both uplinks go to different switches, you want to either look at RSTP or Flexlink for faster convergence.