1what actions were taken by the staff to prevent this in general?
Things like door alarms, baby gates, no classroom doors that lead outside, two people in each room, hall monitor at all times.
2were they aware of your child's unique ability to become Houdini? Or was this a one time event that you couldn't predict?
My sons middle name should be Houdini or at least problem solver. He can get INTO locked closets climb counters desks etc. he has also run from his teachers on several occasions but has only once made it outside to the playground. His goal is predictable. He doesn't do things because he can but rather because he wants it. As a result we have child locks on dangerous closets. A baby gate in the kitchen, and a door alarm on outside doors that he can use. Also sliding bar locks are useful.
3 did they take additional steps to protect your son?
See 1and 2 for suggestions. If they were aware of your child's skills at escaping and did nothing to take extra precautions especially if you volunteered to buy stuff you use at home they are clearly at fault however if they implemented EVERYTHING that was suggested or increased the security measures for his room they may try to pin it on you and your child. If the only way to be certain to prevent him from escaping is a straight jacket that is unreasonable and the daycare doesn't have the resources to do this.. Nor should it be done on a 4y old.
4 how far did he get?
Was the yard fenced or did he wander the hallways so he couldn't get anywhere dangerous? Did he cross a street or wander a distance before being noticed? Who brought him back and from where?
This is possibly the most important question if their idea of protection is a hall monitor and your idea is not opening the classroom door you may be on different wavelengths but if he never actually got outside the daycare can argue he was never in danger however if your child was walking and no one knew he was missing THEY are in deep legal kimchi.

They both non verbal low function autistic boys with no fear of danger

Also what do you do if ur children I hav 2 night wanderers that have gotten up middle of night have broken windows I'm afraid to lay my head down at night

It was a pre school with only 6 total special needs children in classroom and was 3 adult teachers supposed to be watching them and he was found goin down a sidewalk and some guys on motorbikes found them