MTG Layer System - Does adding a basic land type to a land grant the mana ability at layer 4 or layer 6?
When a card has one of the five basic land types it simply intrinsically has a corresponding mana ability:


  305.6 The basic land types are Plains, Island, Swamp, Mountain, and Forest. If an object uses the words “basic land type,” it’s referring to one of these subtypes. A land with a basic land type has the intrinsic ability “{T}: Add [mana symbol],” even if the text box doesn’t actually contain that text or the object has no text box. For Plains, [mana symbol] is {W}; for Islands, {U}; for Swamps, {B}; for Mountains, {R}; and for Forests, {G}.


This isn't granted by the layer system because the layer system is about interaction of continuous effects (and in fact exists inside the rule section named the same). The intrinsic ability isn't granted by a continuous effect; it's just granted by the rules as a matter of fact based on the card type. A card granted a basic land type simply has that ability as soon as it gains the type (at layer 4).

A basic land can lose all abilities though, in which case it would lose these abilities in layer 6.