Article one says what they are allowed to own it for. Artical 4 does not negate that, only says they are allowed to own what the Constitution says they can own. Art 4 is general not specific. Art 1 is specific. Thus Art 1 is one that limits Art 4. To say otherwise negates the plain language of Art 1 specifying what the Federal Government can own land within a state for for.
Article I makes it clear who exercises exclusive control over land and property obtained from the states by act of the state legislature. Article IV makes it clear who exercises exclusive control over land and property belonging to the federal government after a state is admitted. The two are not mutually exclusive.
To say otherwise negates the plain language of Art 1 specifying what the Federal Government can own land within a state for for.
Since you fixate on specifics then why are you ignoring the "purchased by the Consent of the Legislature of the State in which the Same shall be" modifier in Article I? The land in question was not purchased from the states. It belonged to the government before Oregon was admitted and it belonged to the federal government after Oregon became a state.