According to official release notes, the latest AEM 6.4 is 6.4.8.4. In terms of AEM upgrade, yes you will need to upgrade AEM Forms as well. Assuming that you would like to upgrade your AEM to version 6.4.8.4, you will need AEM Forms to be upgraded to version 5.1.212.
I do not think you can skip AEM Forms upgrade - you can try on your own, but even if it will work, this is not recommended approach.
Please find detailed AEM and AEM Forms compatibility matrix to get all information about AEM Forms version you will need upgrade to, depending on AEM version.