Final answer:
In the context of complementary base pairing in DNA, purines pair with pyrimidines: adenine with thymine, and guanine with cytosine.
Step-by-step explanation:
According to the principle of complementary base pairing, purines always pair with pyrimidines. In DNA, the purine adenine (A) pairs with the pyrimidine thymine (T), and the purine guanine (G) pairs with the pyrimidine cytosine (C). Therefore, the answer is 4) Guanine, as adenine pairs with thymine and guanine pairs with cytosine.