Armada 2525 is an early 4X game for PC, predating Master of Orion.
Armada 2525 is fairly simple as 4X games go; the diplomatic options are limited, and victory is a purely military affair. Like many 4X games, Armada 2525 is played on a 2D field of stars, with play going in turns. The player's main tasks are research, building defenses and fleets, and colonizing new systems.
Research in Armada 2525 is interesting. There are eight branches of research: Weapons, construction, hyperspace, force fields, biotech, planetology, info science, and psycho science. Researching these gains you "levels", but doesn't directly gain you any technology.
Technology has level prerequisites in one or more sciences. So, as a hypothetical example, "virus bombs" might require a weapons level 4 and biotech level 7. When you reach those science levels, the tech would unlock. This means that rather than the Civilization model where your empire is pursuing a single technology to the exclusion of all others at any given time, in Armada 2525 technology unlocks as a more organic progression.
Most of the star systems in Armada 2525 are effectively identical, but there are a few "neutron stars" scattered around the map. The player who owns a neutron star (and thus has access to "neutronium") becomes capable of making far more advanced equipment. As a result, there's an incentive to get out and explore as quickly as possible; if you turtle in your home system, you're likely to find yourself facing fleets that are far more dangerous than anything you can field.
Combat is odd, somewhat like a non-interactive version of Space Invaders. The defending system is at the bottom of the screen, the attackers start at the top left, and the defenders at the top right, and you watch the battle play out.
Armada 2525 also supports hotseat multiplayer.
It's a good, fun 4X game that manages to be complex enough to be interesting while remaining simple enough to play in an evening.
Places to Buy
Look for Armada 2525 on your favorite abandonware site.