What is a tiny round white pill with 4 on one side and nothing on the other side used to treat?

Pill imprint 4 has been identified as Nitroglycerin 0.4 mg.

Nitroglycerin is used in the treatment of angina; heart attack; raynaud's syndrome; heart failure; angina pectoris prophylaxis (and more), and belongs to the drug classes antianginal agents, vasodilators. Risk cannot be ruled out during pregnancy.

Please click on the link to view an image of the pill:

https://www.drugs.com/imprints/4-10381.html