I would caution that if your tenants are already under contract on a new construction purchase, which it sounds like if they're far enough along that builder is "pushing things longer than originally planned," they may be more financially locked in than they think, so have them produce a release agreement before getting too involved in your sale.  

The same market conditions that affect large apartment complex vacancies will generally also affect 4-plex vacancies in a down market.Advantages of having a fourplex is limiting downside risk and having peace of mind by locking 30-year rates (as some people have mentioned). 2-4 unit buildings also allow cash constrained investors to get in on the RE game, particularly with techniques like house hacking. 

Sometimes something comes up that is too good to pass up, so use what you can to get it locked down.

Listen to this...pin your ears back and focus on that down payment.

Access gates shall comply with the requirements of Items 1 through 7 above, and shall be equipped to accommodate a locking device.

Don’t leave value locked up and waiting; make your money work for you!

Say you make the loan but are locked in for a period of time prior to being able to access or remove your investment and are now forced to pay back the loan. 

Remove ceiling fansUse non locking handles.

So I market the property as a lease option where, for a fee, a tenant/buyer can lock in a price right now that they can purchase the property for within the option timeframe.  

You can meet them there to evict literally and change the locks.