Hi
Saw that you started with 500 concurrent users, "Best Memory Settings"
https://moodle.org/mod/forum/discuss.php?d=414650. What was the final result?
You must be familiar with the Performance FAQ, you'll see that there is no formula for 'N concurrent users':
https://docs.moodle.org/en/Performance_FAQ#What_are_the_requirements_for_N_users.3F. Still your number, 400, is manageable as opposed to numbers starting from upper thousands we see here now and then. As a very rough estimation in your range
https://docs.moodle.org/en/Performance_FAQ#What_hardware_should_I_buy_for_.27n.27_concurrent_users.3F says, "your Moodle site may only handle as few as 10-20 concurrent users per GB of memory", so 160 to 320 for 16 GB. I would say, you can give it a try.
The success will depend on many factors. Will those 400 (try to) open the quiz during a couple of seconds or a couple of minutes? How heavy is the exam? Does it contain just the standard question types in the quiz module? Will the questions contain heavy images, or even videos? How big is the first quiz page? One question? Ten questions? Fifty questions?
The success will also depend on how well you set up the server. As Ken wrote earlier, having both the web server and the database server in a single machine is a balancing act. You need a good system administrator. Even then, no system admin can set up the server in one go. You have to put it to use, monitor the load during not so critical loads, come back and change settings. May be to add RAM, may be add a caching mechanism like Memcache/Memcached or Redis, etc.
The system administrator can also over do and make things worse. Right now, I am analyzing an overly complicated server "infrastructure". I bet I could get ten times the performance from the hardware if I start from scratch.
