Using only 4 Bytes of AES-CMAC if i calculate a AES_CMAC for 64 Bytes, i get a 16 Byte CMAC.

Is it ok to use only the first 4 Bytes for authentication?

Can i assume that if the first 4 bytes are correct, the other bytes must be ok too?

Thank you!
With AES_CMAC, tag truncation acts like you would expect; if an attacker generates a message and a random tag, then they have a $2^{-32}$ (or one in 4 billion chance) of happening to pick the correct random tag (and getting the message accepted), and with AES_CMAC, there's nothing they can do to improve on that.

Is a $2^{-32}$ success probability per attempt by the attacker acceptable?  Well, that depends on what your authenticating (and the cost to you of accepting an invalid message); in some scenarios, that might be quite acceptable, in others (e.g. doing financial transfers) it might not.