I have a couple guys in their 2nd year of arbitration. Next year, when they have 5 years, and have last year of arbitration, will every player also have an option to do a long term deal? Is it a guarantee that they will have that option?

Trying to decide if i need to lock my guys up now when they have 4 years of service.

Yes, they will still have the long term contract option next season. If you want to keep the player long term, you must offer him a long term contract next season during arbitration. If you arbitrate him a 3rd time, he will very likely choose to become a free agent the following season.
There is one reason to maybe offer him a long term contract in the 2nd year of arbitration; You will sign him at a lower annual salary than what he will want next season. But you end up losing one year of control, so most owners wait until the 3rd year of arbitration to give him the long term contract.