The bore in the jet has 2.0mm.
Before using the 4mm tap did You make a bore with 3.2mm ?
The reason for 3.2mm bore is:
Generally and normally when a thread has to be cut the bore must be:
Thread size mulitplicated with 0.8.
Examplef or a M6mm thread ---> 6mm x 0,8 ===> 4.8mm drill
In Your case: M4mm x 0.8 ===> 3.2mm.
My rule is:
When I have to cut a thread I do this:
Turn the tap three-quarters forward. Then one quarter backwards, ...again three-quarters forward, .... one quarter backwards.
After four or five turns I unscrew the tap, Clean it and blow with air pressure into the bore to blow out the shavings. (safety goggles !!! )
Then again I continue the cutting.
To cut brass is dangerous