[quote=@Archangel89]
Question how plausible would it be to go into the void and come back possessed by a void being?
[/quote]

The closest thing to [i]that[/i] scenario would be someone who is mutating into a void being, which is a painful and maddening process.

[quote=@Bishop]
4th dimension as in that parasyte in "Stranger Things" which had the ability to traverse through dimensions. That creature was a 4 dimensional being. 

By 4D site I imply that my character can see through objects and all around him in a 360° view sphere. Basically an all-seeing power in ever direction. This includes seeing invisible spirits, other beings and of course peek through the void. All this is done at will. Have you seen that Constantine movie with him seeing dead people all around? Or that Dr. Strange movie seeing spirits and spirit parasyte leeching of humans. 

And about the 360° view. My character literally carved out his eyes and went half mad upon entering the void realm and after a prolonged stay. So he doesn't use his eyes to see hence not limited by them as if he were to see 4D with his eyes. He sees 4D with his "inner eye". So he can choose to see what you ate for breakfast, then a second later focus on seeing through you and the house behind you, then he can choose to see inside the house or even behind it.
[/quote]

This is a bit powerful, and it would trivialize parts of the story. What your character could do, however, is to see echoes of both dimensions. That is, things that has recently happened in a place and the corresponding location in the void. But seeing through objects is going to make the 'search for this person' part of the story more or less pointless.