Bought 3 roku4's within a couple months of original release 4 years ago, one was replaced after cleaning misapp 2 years back; units were eventually upgraded with 32GB microsd cards and ran fine for well over a year after. After firmware 9.2 was pushed out, units began failing one after the other some 2-3 months later, only fix was removal of the microsd cards. Roku advised replacement of microsd card(s) with 2GB 'confirmed' good cards they sold. Fyi, 32GB cards removed were tested and found to be defective. Roku 2GB cards were ordered for all units and have been operating without problems for ~1 year until today, now I'm starting to get defective cards once again. I do own an ultra 4640 unit that has had no problems (no memory upgrade and doesn't complain with the same number of apps as the roku4's) so I'm coming to the conclusion that either roku4's 'eat' microsd cards or firmware updates are involved. None of my units has recieved 9.3 upgrade as of yet, although it was announced quite a while back.
I will keep my eye on 'Sale' specials on newest ultras or take the plunge to fire boxes as roku seems to be getting wobbly.
The need for the SD card isn't really all that critical. It is only used for channel storage, and only if you have a large amount of channels installed. The Roku keeps the most used channels in storage, and if you don't have the SD card it might have to reload the unused channel again if it's requested. Unless you are always using a large number of channels, it's unlikely you'd even experience that reload very often. I've never used an SD card in any of my devices that had the slot. At worse it adds a bit of extra time to load an occasional channel.
The Roku 4 has 512 MB of channel storage, which is the same as all current Roku devices. The Ultra 4640 was the only player that ever had more. As I said, unless you have many, many channels, and constantly use a large number of them, having an SD card will likely never make a difference in actual use.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.