You can use your Moto E4 to format a microSD card as either Portable (the card's default) or Internal. But the phrase, 'default storage' isn't a file system formatting option, that's a reference to apps using a the card's storage media instead of the default internal storage media. But there a lot of other variables involved with what you may or may not be trying to do. You do need to take the time to examine all the relevant issues you'll encounter when you opt to use your own preferences other than the defaults.
-- If you want to use your microSD card to increase the overall storage capacity, use your Moto E4's Settings >> Storage menu to format the card as 'Internal'. It's really, really important you read the messages that pop up, don't skip over them (most people do). For your E4 to keep running smoothly, you need to keep in mind that once you set a microSD card to be Internal, that makes a very significant change to the card's file system. When Internal, the card is re-formatted to be ext4, and it gets encrypted. At that point, the card is a direct match to your phone's internal storage media -- both are now using ext4 as their file systems, and both are encrypted with both encryption keys tied specifically and only to your particular phone. The card is also now virtually an added component to your phone's internal hardware. The installed Android operating system considers your internal storage media and your microSD card's media as merged into one now larger storage media. It's your OS that maintains and manages what gets does or doesn't get saved to your card -- you don't interact with it directly any more. And you must not just physically remove the card to use it as a transfer media, again, you phone sees this as part of your storage media. (... plus, it's formatted as ext4 and encrypted so a Windows nor a Mac computer can view the card's content anyway as neither support ext4 and more importantly, the encryption won't allow the card to mount.)
So basically opt for Internal but be sure you're aware of all the inherent issues that are involved. If you change your mind later, you can reformat the card back to Portable but keep in mind that the longer you use it as Internal the more likely it will be a very disruptive process to change the card back. You'll be able to use the card again but you'll also be likely to lose some app data and such.
-- Most people just leave the card as Portable, that's its default. When portable the card is using either FAT32 or exFAT as it's default file system. FAT derivatives are not very robust or reliable but various operating systems (Android, Windows, MacOS, etc.) include at least rudimentary support for FAT file systems. So a microSD card can be used to transfer files between all kinds of different devices -- computers, mobile devices, web cams, etc.
-- Default storage is just a feature that's included in the Android operating system. It's typically only available on a per-app basic. A lot of Camera apps allow the user the default save folder from being in internal storage to the card, so do some streaming video apps and some multimedia apps. But it is an option that some apps can and most can't do.
There's also an option in your Settings menu where you can 'move' some apps to the card to free up some storage space in the internal memory. It's only indirectly relevant to that 'default storage' And it also is limited in actual usage as most apps cannot be 'moved', it's more of a misnomer as none of the apps are actually relocated completely, a more accurate term would be 'split'. This can free up some storage space but for those apps that can be 'moved', part of their code has to be left within internal storage. 'Moving' an app splits the app's installed data files so some remains in internal storage and some resides in the card.
