Suggestion about sector expire algorithm

Sector expire between n+4 and 4n+4 caused too much replot at beginning

It is done that way for a reason and more frequent replotting initially is an expected behavior.

What about change it to n+4 and 4n+16? Maybe n+8 and 4n+32 would be better

I recommend you to read resources mentioned in What triggers a sector to start replotting to understand why it is absolutely crucial for the protocol and why is not better for network resiliency to change the way that you suggest (of course you’ll be happy to do less frequent replotting, that is clear) unless you can prove that it is.