Nether Shadow

Since it enters due to triggering at the beginning of upkeep, it is not possible to get an infinite loop with four Nether Shadows.

Note that bringing the Shadow back onto the battlefield from the graveyard is not a spell, it is an ability. It can't be countered with something that counters spells.

The last thing that happens to a resolving instant or sorcery spell is that it's put into its owner's graveyard. Example: You cast Wrath of God. All creatures on the battlefield are destroyed. You arrange all the cards put into your graveyard this way in any order you want. The other players in the game do the same to the cards that are put into their graveyards. Then you put Wrath of God into your graveyard, on top of the other cards.