You need to find out what RAM your computer is capable of handling before you just start putting RAM into it, even if the speeds are the same, it's never to much trouble to be careful and do a bit of research to make sure things go smoothly.
Thankfully there are a lot of websites that have already tested these things for you so that you don't make any mistakes. I usually go to:
http://www.crucial.com/store/drammemory.aspx
They have a system scanner service that will scan your system and check your settings, it will redirect you to a page where you can buy RAM, you don't have to obviously, but it will show you all the RAM configurations that your system supports. It will let you know the max RAM capacity, if you don't see 4GB as an option, then it doesn't support it.
If you don't want to use the scanner you can also just input your make model year and cpu to get the same results, though the scanner is usually more accurate if you aren't sure which machine is yours. They do tests to make sure the RAM is compatible and list all the speeds and configurations for the RAM so you know which ones to get. Make sure everything matches up before hand.
Knowing these things will help you find out if the RAM you have or are about to purchase will work so that you don't waste your time. I hope this information helps you out.
You have to decide what you want in terms of storage, performance and OS since there is no option to have 2x1TB drives in the $799 model and there's no option to have a 1TB Fusion Drive in the $999 model.
Essentially, you're paying $200 more to get OS X Server and for the ability to have a second hard drive (doing it yourself is an option if you can follow guides on iFixit and have the tools and accessories required).
Choose the $799 model with:
- A 1TB Fusion Drive - $250
- OS X Server - $19.99
Total: $1068.99 (but you'll have a faster drive but half the storage space of the default $999 model).
Choose the $799 model with the default 1TB drive:
- add another 1TB 2.5" 5400 rpm drive - about $80+ (approximate cost from newegg)
- OS X Server - $19.99
- The Mac mini Dual Hard Drive kit from iFixit - $49.95
Total: $948.94
Hassle of doing it yourself: Priceless
Best Answer
You can find a guide on iFixIt, summarized on MacRumors.com: 2018 Mac mini Teardown: User-Upgradeable RAM, But Soldered Down CPU and Storage
Memory: Yes*
Apple specifically mentions on the Apple Website that the Memory are SO-DIMM sockets (2666MHz DDR4 SO-DIMM).
*However, the warranty is voided if upgraded by the user as the modules are no longer directly accessible from the bottom but instead involve removing the antenna plate, fan and mainboard.
Storage: No
Storage is (like on the MacBooks) soldered onto the logic board and therefor non-replaceable (you would need to replace the entire logic board).