Rain Of Fire (4x07)
"Rain Of Fire" was the episode in which "Angel" truly became apocalyptic. It was an episode which raised the bar in every single way.
As Angel and the group deal with a sudden wave of paranormal activity all over Los Angeles, the beast rises from the bowels of the Earth to bring a “rain of fire” over the city as phase one of its plan to destroy the world. When the group around Angel faces the beast for the first time, they've got to notice that the beast is superior to them in every single way, as they fail to stop the beast from creating the rain of fire.
The fight sequences in that episode are just awesome and just the matter of fact that Angel and his gang were not capable of stopping the beast creates more suspense than the show has ever created before.
Habeas Corpses (4x08) ´
Connor pays a visit to Wolfram & Hart where the indestructible Beast from Hell follows him there and attacks, killing most of the staff. Lilah manages to escape with a little of Wesley’s help while he leads Angel, Gunn, and Fred into the locked-down offices to find Connor which gets worse when all the victims of the Beast become reanimated zombies out to kill.
When even the evil characters on the show get killed, you've got to be prepared for horror and you just have to notice how big this thing really is. Wolfram & Hart, Angel's adversaries since the premiere of the show, easily get executed in this episode, something that you would never have expected to happen so early into the series.
Home (4x22)
In "Home", the crew of Angel Investigations gets an unexpected offer by Lilah who resurrects from the dead. As a thank you-gift from the senior partners of Wolfram & Hart for ending world peace (which was brought by a character named Jasmine in the previous episodes, but only by enslavement and the loss of free will), she offers them to take over the L.A. office of Wolfram & Hart, which got rebuilt since the beast has destroyed it in 4x08. Angel and his crew are tempted by her offer. If they approved her offer, they'd have more ressources to save the people in need of help.
"Home" is an important episode for the show, because it's changing the whole premise of the show and deals with how good things can offered by someone whose actions and opionions are despised. It also sets up the new theme for season five: How can someone survive in the corporate world without being corrupted.