The Raspberry Pi 4 is the first model to offer different amounts of RAM. You have options of 1GB, 2GB, or 4GB depending on how much you’ll need to do what you want to do. Each tier has its own price ranging from $35 to $55. While there isn’t a crazy big difference in price, you still want to make the right choice. The easy answer that works in all cases is to buy as much as you can afford.
If you plan to use to your Raspberry Pi with a display and interface with it directly beyond the initial setup, buy the 4GB version. There isn’t any independent video memory on a Raspberry Pi, so the total system memory is shared for video. Once the system memory is divided up and reserved for the kernel, modules, the RAMdisk/ZRAM scheme your operating system may use and the video frame buffer, you’re left with a chunk of RAM that is set as unusable for any apps or programs.
You’ll be able to get away with a 2GB version, but not if you plan on using a 4K display. If that’s the case, totally go for the 4GB version – it will be totally worth that extra $20.
If you aren’t planning on attaching a display, you may have no problems using the 1GB version. This depends on what type of services you plan on running. A small and simple project, for example, an LED lighting controller won’t need much RAM at all. Something like a media centre, file/print server or a Wifi access point will benefit from more RAM.
You can do a lot of things with a Pi, ranging from small projects to a complete desktop PC that can play games. Most of them will benefit from extra memory, so buying the 4GB RAM version is always the safest and best bet, but if you have a use case that isn’t going to need to be processing much data and you won’t be attaching a monitor, the 1GB RAM version should work and you can save some money.
What would I do? I’d go for the 4GB version every time. You never know how you might want to re-purpose the Pi, grow the project and things in the future.