Wydaje mi się, że jednak będzie Present Perfect Continuous. O ile dobrze zrozumiałam zdanie "on tu pracuje od 4 miesięcy odkąd zachorował pewnego poranka jak zaczynał pracę", czyli Present Perfect, który się charakteryzuje przyczyną w czasie przeszłym i skutkiem w czasie teraźniejszym. Tzn. przyczyną jest choroba, a skutkiem praca. Mam nadzieję, że dobrze trafiłam w kontekst. ;P

Myślę że jednak będzie Present Perfect Continuous

I rethought that sentence one more time and my conclusion is that everythind depends on the CONTEXT because look: "He HAD been working there for four months when he became ill", meaning that he no longer has the job because of disease.
But we can also say that "He HAS been working there for four months when he became ill", meaning that he managed to stay healthy for 4 months and then he became ill although he didn't stop working there! it's something like "She HAS been pregnant for four months when she became ill" - meaning that she was in 4th month of pregnancy when she got ill and she STILL IS pregnant, so we cannot say "She had"... ;)

A ja bym się zgodziła raczej z tokiem myślenia Madzialeny, a zwłaszcza z pierwszą proponowaną wersją rozumienia zdania, czyli do momentu zachorowania pracował tam cztery miesiące, a potem już nie - taka sytuacja wydaje mi się i bardziej prawdopodobna, i bardziej logiczna. Zwłaszcza, że zachowujemy wtedy zasadę następstwa czasów w j. ang., z której wynika, że nie powinno się używać czasów teraźniejszych (w tym PRESENT Perfect Cont.) do mówienia o czynnościach wcześniejszych niż te wyrażone czasem przeszłym (tu Past Simple).

Myślę, że obie formy byłyby poprawne, wszystko zależy od kontekstu.

Madzialena0319, because you started speaking in English, I will follow your choice. I took some advice from a few experts, and I can say

He has been working there for four months since he became ill one morning just as he started his day's work.

in your opinion, it means that he managed to stay healthy for 4 months and then he became ill although he didn't stop working there!"

I think you incorrectly understand the function of "since". We use since to indicate when something started in the past.

I've been reading this book since last week.
(I started reading this book last week, so I've been reading it for 7 days now)


Let's now come back to your sentence. Having known what "since" means, we can say the sentence means "he has been sick for the last four months while he has been working there."