Thank you all for your comments and inputs. I really appreciate your time and effort. I will take them into consideration.
I just wanted to clarify something here. I probably confused a little bit about the one month security deposit that I planned to cover tenant's rent. I didn't change the contract yet allowing them to use their one month security deposit to cover their rent. I just sent them an email saying I am willing to negotiate with them by giving them one month of their security deposit to be used for last month's rent (but not for March's rent). Because they never responded to my email, call & text messages.
This is what I told them in the email:
....."Please be advised, as explained in my previous email, I will only keep 1 month security deposit, instead of 2 months as per lease agreement. You still need to make a rent payment for March but you don't need to pay last month (April) rent.".....
However, since they didn't pay their March's rent until today, I was then thinking to send them an email saying that I would apply the one month security deposit to cover March's rent instead of last month's rent. But I haven't sent the email yet to them regarding changing the month to cover, March's rent instead of last month's. I don't know if there is any difference in changing which one month to cover by security deposit, last month or the month before.
I arrived with a decision of giving them their one month security deposit to cover their rent also because I had a discussion with a friend of mine who's also an investor and told me he only keeps 1 month deposit, never 2 months deposit.
So after thinking it through, since they have been good tenants for 4 years, it's probably not a bad idea if I negotiated with them that I would allow them to use their one month security deposit to pay for their last month's rent (not for March's rent).
Yes, I am really thinking how this cost the least to me. The reason why I didn't really want to go with the eviction, especially when they have been exceptionally good for 4 years, it's because the time, the cost and the risk involved. According to the RE agent I spoke with when I was talking to him about the possibility of selling the house and mentioned my problem with the tenant, he said that it could cost me around $1,400 for eviction, which is equal to my one month rental income. The tenant might get upset because of the eviction and might destroy the house. I don't really know if they would do it, but there is a possibility. I know there's always a risk involved, but I want to minimize the risk as much as I could.