ruby methods .map, .each, .gsub changes overview for puppet v3 & v4

Can anyone give me a good overview of the changes between ruby in puppet v3 & v4.
In particular the methods named above.

I need to cvt our p3 (3.2.2) system to p4, whilst at the same time, ensure that our code will run against both versions of puppet until we have fully converted.

At the moment, I frequently come across msgs like Detail: undefined method `gsub' for nil:NilClass for the above methods, and have to recode them.
This is doubly tricky as I know nothing about ruby ;)

Any help gratefully received

Chris