My cuttings are doing great, but are still in their bin.
I won't be at home for 4 days and that's why I' m worried about them now.
Should I keep the bin closed for 4 days (too much humidity perhaps),
leave the bin open the whole time ( the fragile leaves might get dry),
leave the lid slightly open, or else ask the neighbor to come in and
open the bin for a short moment once a day?
What would you do in my place?
Thank you in advance for your kind advice.

Definitely have a friend come in and teach them what to do.

If the neighbor is willing, go for it. Other wise leave them closed or very slightly open. More risk from too little humidity than from too much, i would think.

I have the exact same issue. They have to fend for themselves for 4 days. I'm leaving the humidity bin cracked 1/4 inch, then pointing a tiny desktop fan at the crack in the bin. Fan is connected to a digital timer that is set to turn the fan on for 1 min twice a day. I figure that will give the air exchange I need. Cost was $15.

I had mine in bin for a week (8 days) with nobody tending.  Here is what I did:

Gave each a little water.
Put a pie pan of water under the wire rack in the bin.
Propped the lid open slightly to allow air exchange and stabilize humidity.
Also, it is in a window with supplemental light @ 12 hrs/day.

When I got back from my trip, they were all healthy with new leaves and lots of root growth.

If its not a burden, ask your neighbor for help. 

Is it possible to take them with you? I took my bins of cuttings with us when we had to drive out of state to see family. Yes, my wife and everyone thought I was crazy. Yes, I had venting bins in our room's closet and shower. And indeed, it was more hassle than any of us needed. Crazy ... perhaps. But the cuttings survived. It is merely an example of what this fig thing does to the mind.

Thank you everyone for your advice. That was very kind of you.
Luckily, my neighbor is willing to look after my cuttings, so I can be
quite sure that they'll survive (fingers crossed).