When do you use שתיים as opposed to שתי?
Just the number is שתיים
If it's a quantifier of some objects it's שתי. If it's a quantity, but the object isn't mentioned you use שתיים.
Like שתי can mean a pair?
When would one put emphasis on the last syllable of ארבע versus on the first syllable?
Spoken Hebrew: first syllable. Formal Hebrew: last syllable, if the speaker is sufficiently educated to remember that this is where it "should" be...
יש תרגילי חשבון למתקדמים?
Arba ze lo pakhot mi-shtaim.