It’s been a while since the last update/review. And this one I am going blind on as I do not have the units to review. But I have seen the reviews of others to work on. So here goes...
 
The Raspberry Pi 3B+ and 3A
 
The R-Pi 3B+ is faster and improved version of the R-Pi 3 (aka R-Pi 3B), with Bluetooth 2.0, improved Wifi connectivity (fixed a bug where the R-Pi 3B would randomly disconnect from the network), and slightly faster CPU. Like the R-Pi3, it can boot from the USB Port. It is the last of the R-Pi Model B/B+ series.
 
The R-Pi 3A goes back to the R-Pi A+ series (none were made for the R-Pi 2 series). The Model A/A+ is half the RAM of the Model B/B+, no Ethernet and only 1 USB Port. Despite what one would think as limitations, it is still a useful unit for various sets from robotics to a standalone system. The appealing part of the Model A/A+ was its price: $10 - $20 off compared to the R-Pi Model B/B+’s price. The Model R-Pi 3A has 1GB of RAM, and has the same smaller footprint size of the Model A+. Like the Model 3B/3B+, the 3A does have Wifi and Bluetooth and it can boot from the USB Port though with only 1 port, it would be difficult to do without a USB Hub. Price: $25.
 
Unlike the R-Pi 0/0W, the R-Pi 3A has a switching power supply section to filter the voltage inputs, and a self-healing fuse to protect the R-Pi from surges, just like on the other Model A/A+ and B/B+ series. The R-Pi 0/0W offers no such protection, and like some have discovered, you can easily kill it with over-voltage.
 
The R-Pi 4 is a totally new system though it is based on the R-Pi 3 with several modifications.  The footprint has been changed despite it being the same size and options of the R-Pi B+ Series. On the Model B+ Series, the 4 UBS Ports are on the left side of the board and the Ethernet Port is on the right; on the R-Pi 4 the ports have been switched to the 4 USB Ports on the right and the Ethernet on the left. Furthermore, the 2 USB ports on the center part of the Board are USB 3.0 and the 2 USB Ports on the extreme right are USB 2.0. Because of this port location switching, cases for the former Model B+ series will not fit the R-Pi 4. You would have to get a case that is specific to the R-Pi 4.
 
Oddly, early models of the R-Pi 4 cannot boot from the USB port directly like the R-Pi 3. More on this later…
 
Before May 2020, the R-Pi 4 came in “3 flavors:” 1GB ($35), 2GB ($45) and 4GB ($55) Ram sizes. They all have the same options of the Model 3B/3B+: 4 USB Ports though two are USB 3.0 and two are USB 2.0, camera and display ports. However “Improvements” are – 2 HDMI Ports for Dual Monitors, the USB 3.0, and faster CPU. Introduction of 4GB allows for more complex applications to run faster on the system, where a before the system would use some drive space as virtual RAM. But with the new 3.0 USB Ports, direct booting from the USB Port has been removed, indirect booting from the USB Port is still an option however (booting from the SD Card first and then moving to a drive on the USB Port). With the new system usually means a new Raspbian Distribution release, in this case it is now called Raspberry Pi OS (version Debian Buster).
 
After May 2020, the Raspberry Pi Foundation continued the R-Pi 4 with 3 “flavors” but discontinuing the 1GB unit and making the 2GB unit as the bottom line system (for $35) and the 4GB unit as middle system (for $45). A 8GB system now is the top of the line system for ($55).
 
With 8GB, the R-Pi4 can run 3D applications without problems or issues as they had the in the past (like glitches and slowdowns). It is capable of processing 3D Printing and CAD programs. It is more like a full desktop system compared to a Windows system many times its price
 
With these later models of the R-Pi 4, booting from USB has been restored, making it capable of booting from hard drives and SSD connected to the USB Port.
 
Power, price and compatibility, both the R-Pi 3B+ and the R-P1 4 series.