What makes you think DLC is just a 4th OF?? He's been great so far. Defensively he is great. If he doesn't have enough power, he can play CF and be atop of the order bat.
Jesus Sanchez is one of our top prospects and has been a little up and down, but overall he's played well. He's a 23 year old rookie. He will definitely get more consistent with more AB's.
I would still try to go after Marte in the off season, but I just don't think it's as terrible a situation as you think. Sanchez and DLC are an upgrade over Dickerson and Duvall who were our starting corner OF this year.