Questions & Answers about Ja mam cztery, a ty pięć.
Why do we use the pronouns ja and ty here if Polish usually drops them?
You are right that Polish usually drops subject pronouns like ja (I) and ty (you) because the verb tells us who is acting. We include them here to show a strong contrast between the two people: I have four, whereas YOU have five.
Why does the sentence use a for 'and' instead of i?
The word i is used for simply adding things together. The word a is used when comparing or contrasting two different things or situations. Since we are comparing what I have with what you have, a is the natural choice. It translates well to 'and', 'but', or 'whereas' in English.
Where is the verb for the second part of the sentence? Shouldn't it be a ty masz pięć?
You can absolutely say a ty masz pięć. However, Polish often drops the second verb if it is the exact same action and is already understood from the first half of the sentence, just like we sometimes do in English.
Why do the numbers cztery and pięć stay in their basic dictionary form here?
In this sentence, we are stating the numbers on their own without putting them next to a specific noun. When you are just giving a standalone quantity or counting abstractly like this, you use the basic, unchanged forms of the numbers.