Learn Croatian: Auto ima četiri nova kotača i dvije rezervne gume. - The car has four new wheels and two spare tires.

After the numbers 2, 3, and 4, Croatian uses a special paucal form. For masculine nouns like kotač, the noun takes the genitive singular form (kotača), and the adjective takes an -a ending (nova). If the car had five wheels, it would trigger the genitive plural rule instead: pet novih kotača.

For the feminine noun guma, why is it dvije rezervne gume?

For feminine nouns, the rule after 2, 3, and 4 is much simpler because the noun and adjective look exactly like the standard nominative plural. Guma becomes gume, and the adjective rezervna becomes rezervne.

Why is the word for two dvije here, but the word for four is just četiri?

The number two has different forms depending on gender: dva for masculine and neuter nouns, and dvije for feminine nouns like guma. The numbers three (tri) and four (četiri) do not change based on gender.

Since the car has these items, shouldn't the objects be in the accusative case because of the verb ima?

The verb imati does normally require the accusative case. However, the numbers 2, 3, and 4 lock the following words into their special paucal forms. These forms do not change when they are the direct object of a sentence, so you don't need to apply the usual accusative endings here.