Is it recommended to format an external drive to ext4?

I think the core part of your question that is important is

I will only use it on two Ubuntu machines.

I faced the same dilemma as you and chose EXT4. The primary reasons I chose EXT4 are because 1) EXT4 will preserve my file permissions and 2) EXT4 better manages the storage of the data on it (e.g., it doesn't need to be constantly defragged and "coddled" like NTFS does). EXT4 is a more self-reliant file system than NTFS.

The only negative to using EXT4 is that you won't be able to access the data on it via a Windows machine. But as you said in your question, this won't be a problem because you are only using Ubuntu machines.

IMHO, the only reason to use NTFS is if you need to access the filesystem with Windows. If you don't, NTFS is really an inferior file system.

If you will be using it only on those two machines and plan to be religious about properly unmounting the drive before disconnecting the USB/powering it off, then yes, ext4 is highly recommended.

• ext4 will give you improved performance over ext3, particularly when trying to write multiple files at the same time or when creating a large file.

• The tradeoff is a slightly higher chance of data loss if the drive is not properly unmounted.

I'd recommend it. ext4 is the default file system for Ubuntu for good reason. As a side benefit, you can use the Disk Utility to encrypt the entire drive with a very strong password. That way if you ever lose the external drive (or if it is stolen), your data can't ever be accessed by a third party.

I use ext4 on all my drives, internal and external. For those drives that need to work with a friends Windows machine, I'll have a small FAT or NTFS partition.