I've been working on this picture little by little over the last four months and now it is completed.
It has taken me four months, little by little, to finish this painting.
ーI've been working on this picture little by little over the last four months and now it is complete.
「４ヶ月かけて少しずつこの絵を書いていましたが、やっと仕上がりました。」
I've been woking on this picture「この絵に取り組んできた」
little by little「少しずつ」
over the last four months「この４ヶ月の間」
ーIt has taken me four months, little by little, to finish this painting.
「この絵を少しずつ描きあげるのに４ヶ月かかった。」
It has taken me four months to ... で「私が…するのに４ヶ月かかった」
finish this painting「この絵を描きあげる」
ご参考まで！
カナダ 
