It isn't upgradable and even if it was it wouldn't be worth it.
Lots of things can run on 4MB of RAM and it should be more than enough for simple wordprocessing and compiling small console programs.
How long have you been using computers for?
Most people nowadays don't use even half their computer's power and if they do it's wasted on running anit-virus software. Software is becomming more bloated, and I'm not just talking about Windows, OpenOffice and Linux are suffering from feature bloat too. Do you realise that as long as you're not playing any games or ripping any DVDs the amount of memory in your PC that's actually being used to store you data is minimal?
You can do a lot without using much memory, 10,000 word essay only takes 64KB to store and even a 2560x2024 16M colour image only takes 15MB! So I hope you can imagine that my 4MB laptop is way oversized for word processing, and my old p200 with 32MB of RAM is more than big enough for saving images I've taken with my digital camera providing I install fairly light software on them. I rest my case, for most simple computing tasks like web surfing and word processing with moderate sized images 16MB of RAM is probably enough for most applications.
EDIT:
I've downloaded all the DR-DOS images, now I've just got to get hold of some floppies and install this thing.