I have one of my site was adding a new NAS, but after successfully adding it like 12 hrs later the router reboots, have tried replacing with a new rb4011 still it rebooted. could you assist with this. find attached the error
Congrats on putting the whole topic content as the topic Title.
Anyway, out of memory condition was detected is pretty self-explanatory for your problem.
Someone added a new NAS device to the network and now the rb4011 keeps rebooting because of that event? Interesting.
Fixed
NAS is completely irrelevant (or adding it may have "triggered" something...)...
You're experiencing a DDoS attack, and your RouterBOARD is killing itself due to the stupidity of adding the attacking IPs to the memory.
In fact, the memory runs out, and the machine crashes.
In fact, the log says the device is running out of memory...
If I'm wrong, well, you deserve it.
You haven't provided any relevant details, so just as your question is inappropriately worded, so is my answer.
I will try to summarize what @rextended is trying to say:
https://mikrotik.com/consultants
Cheers!
LoL, only ONE in Latvian 
Shame Mikrotik! 
Latvians don't need consultants; they understand Mikrotish by nature.
Naaah, it is a learned skill, they have since secondary school (when the kids are around 11 years old, and very receptive) 3 hours/week of Mikrotikish in public school, in three different days, usually after Vogon Poetry for Documentation and before the one about Applied Confusing Tables (on fridays this latter is replaced by one hour of Telegraphic Writing).
It's a bot, it wrote exactly what I wrote above, in other words....
Not sure if you flagged it, but I have.