To reply to your initial question I would say 4 KB is the maximum block size that you can use. Have a look at the following link: http://en.wikipedia.org/wiki/Ext4
As far as inode is concerned it is basically a term used for index-node. inode keeps a track of files / directories hierarchy on a file system. It also keep information about files / directories attributes like time stamps, location etc. Default inode size for ext4 file system is 256 bytes. You can increase the size of the inode to accomodate additional attribute that it can keep track of. inode also defines the maximum number of files that you can have a on a file system.
For me inode is basically a feature or algorith that keeps track of indexing and metadata of files/directories on a file system.
If you want to find out the current inode size on a particular partition you can use the following command:
tune2fs -l /dev/sdxx
Where first x will be the slot on which your hdd is connected and the second x will be partition number.