Completing undergrad in 4.5 years
Extra time you spend doing research will almost certainly do more to help your chances of admission in graduate school than taking >4 years to graduate will hurt - especially if you don't have other strong research credentials (with the caveat that admissions committees are full of people, and you might run into one person who for some reason decides that taking >4 years to graduate is the mark of a failure - it isn't really worth your time worrying about each of these possible types of people).

However, you should keep in mind that graduate admissions, particularly for PhD programs in the US, typically occur only at one time in the year, for fall semester, so you should have a plan for what you intend to do the rest of the year. If you can get hired in a university lab in your field of interest, that would be ideal for graduate admissions.

This answer is based on my experience in Biological/Biomedical Sciences in the US, and advice may vary by field and country; I also have in mind primarily PhD admissions.