You could try upgrading the ram to 8Gb and if you want the HDD to a SSD. If you dont want to do that, you could try doing a factory restore. Im not sure how cluttered the mac is but the restore will remove everything and make it run faster. If you are going to do the restore, make sure you save everything you want to before doing it. You probably should still upgrade the ram to 8GB anyways but if it runs fine after the restore you should be ok for atleast a year or 2.
[http://www.tomsguide.com/faq/id-2330399/restore-macbook-pro-factory-settings.html]
That guide should help you complete the factory reset if you choose to do so.
Hope this helps, if it did please hit the accept answer button!