Yes, correct format of SSD is ext4.
But that is done also when you install first time Umbrel.
So if the drive is brand new (no partitions) it will be formatted in ext4 in the process.
Formatting it in ext4 from the beggining it doesn’t hurt, but users should keep in mind: do not format it in NTFS or using Windows systems. That will make unusable the disk and you will need to re-format it in ext4.
In the troubleshooting manual there’s a procedure mentioned (for when you want to copy the blockchain data from another source) about how to use ext4 disks on Windows.