Do you change your pronunciation depending on the sentence?

Take this example for instance:

I'd like half of that!

and

It's half past four!

In the first sentence I would pronounce half properly, but in the second sentence I would say "haf".

In both sentences "half"is pronounced /h@f/ or /haf/.
I think there is difference between intonation of two sentences.
In first sentence, stress is placed on "half", but in second one "half" is unstressed.

<<I'd like half of that!

and

It's half past four!

In the first sentence I would pronounce half properly, but in the second sentence I would say "haf". >>

Yes, I tend to do that too. Essentially, "half" has two pronunciations, /haf/ (same vowel as "cat") and /hA:f/ (same vowel as "calm"). When saying "half past" I generally use /haf/; in other contexts I can use either but probably more often /hA:f/.

I'm from the north of England, so "past" definitely has /a/ not /A:/, and I wonder whether it's influencing the vowel of "half" in that context.

JHJ- Might just be a Northern English thing, we'll have to see what others say!

Well, I do exactly the same thing, but I'm from oop North too, so that doesn't really help, does it?? :-)
I'm from the Lake District BTW, and lived in Manchester for a few years.