Is major zero in Metal Gear Solid 4

Yes, Major Zero does appears in Metal Gear Solid 4, but in a post-credits scene. He is shown as a "vegetable" in a wheelchair as he is 105 years old in this game (he was born in 1909 and MGS4 takes place in 2014, so it takes place 5 years after the Big Shell Incident (MGS2 - the one where you play as Raiden) and his 100th birthday. The reason why he's a "vegetable" in MGS4 is because due to his age, he is in a persistent vegetative state (or simply put he's in a somewhat permanent coma).

The reason he is in the ending of MGS4 is because Big Boss (AKA Naked Snake from Metal Gear Solid 3: Snake Eater, Metal Gear Solid: Portable Ops, and Metal Gear Solid: Peace Walker), found him through the means of AI's (Artificial Intelligent proxies that were supposed to carry on Zero's will, but failed) and wanted to make loose ends with him.

As Big Boss wanted to show his "son", Solid Snake (known as Old Snake due to his accelerated aging because of his status as a clone from his father, Big Boss as a result of the Les Enfant Terribles (A.K.A The Terrible Children - The project designed to clone super soldiers from Big Boss, which later conceived 3 children - Solid Snake, Liquid Snake, and later Solidus Snake)) Zero and tell him about their bitter history of each other (due to each person having a different interpretation of The Boss's Will - Ex: Zero wanted power over the world while Big Boss dreamed of creating a military nation where soliders do not take orders under a country - Outer Heaven).

After reflecting on their past experiences, Big Boss switched off Zero's life support equipment and Zero was struggling to breath which Big Boss pressed his hands over Zero's heart to kill him, ending the dictatorship of the organization he created and putting his former friend to peace.