Any other effects of that damage, such as life gain from lifelink, still apply.

If Unctus's Retrofitter leaves the battlefield before its enters-the-battlefield ability resolves, that ability will have no effect. The target artifact won't become an artifact creature at all.

Multiple instances of toxic are cumulative. For example, if a creature has toxic 2 and gains toxic 1 due to another effect, combat damage that creature deals to a player will cause that player to get 3 poison counters.

The resulting artifact creature will be able to attack on your turn if it's been under your control continuously since the turn began. That is, it doesn't matter how long it's been a creature, just how long it's been on the battlefield.

The target artifact retains any types, subtypes, and supertypes it has.

Damage dealt by a creature with toxic grants the same number of counters regardless of how much damage is dealt. Notably, if a replacement effect modifies the damage in some way (such as that of Gratuitous Violence), the number of counters given remains unchanged.

Unctus's Retrofitter doesn't remove any abilities the target artifact has.

Conversely, replacement effects that apply to the number of counters put on a player can modify the counters placed this way. For example, Vorinclex, Monstrous Raider's last two abilities can apply to counters placed this way.

Effects that modify a creature's power and/or toughness, such as the one created by Blazing Crescendo or a +1/+1 counter, will apply to the creature no matter when they started to take effect.

Toxic doesn't change the amount of combat damage a creature deals. For example, if a 2/2 creature with toxic 1 deals combat damage to a player, that creature will deal 2 damage. The results of that damage are the player loses 2 life and gets a poison counter.

If a creature with toxic deals combat damage to a creature or planeswalker, or if it deals noncombat damage, toxic has no effect and no player gets poison counters.

A player with ten or more poison counters loses the game. This is a state-based action and doesn't use the stack. In other words, it happens immediately and players can't respond to it, just like a player losing the game due to having 0 or less life.

If the artifact was already a creature, its base power and toughness will become 4/4. This overwrites any previous effects that set its base power and/or toughness to specific values. Any power- or toughness-setting effects that start to apply after Unctus's Retrofitter's ability resolves will overwrite this effect.