Here is a description of all the endings and how to attain them.

First Ending: Goto the castle tower and defeat Richter without meeting Maria. Alucard will escape and say a few things before going back to his eternal sleep.

Requirements to get endings 2-4 (Holy Glasses): You need the gold ring (after beating the Succubus in Nightmare, located in a side path in the long tunnel to the underground) and the silver ring (in the hall of spikes in the tower, you need the Spike armor from the underground.) Take these to the clock tower and equip them, go down and follow the path to find Maria; she'll give you the Holy Glasses. The third and fourth ending require you to have no defeated Richter(by destroying the green orb above his head while wearing the Holy Glasses).

Second Ending: Get the Holy Glasses and defeat Richter. Maria will have also escaped and tell you she's continuing her search for what happened to Richter.

Third Ending: Defeat Dracula and you'll see Maria and Richter waiting for you outside the castle. Maria will stay with Richter

Fourth Ending: Get near 200% completion and defeat Dracula. Richter and Maria will be waiting but this time Maria will follow Alucard.