Set up as 4 x 6 and in the print dialog set the paper size to 8.5 x 11, set the position to centered, and add the marks in the marks and bleeds section.
I want the pdf to be set up in the center and with crop marks, so i cant send it to a print place.
You don't select a page size when exporting to PDF, but you can add the crop marks in the export dialog.
Acrobat and Reader will both automatically center the PDF when it is printed, but it's up to the print shop what size paper they use. Make sure they don't scale the print to fit the sheet.