The longest-lasting form of vitamin K2, with a half-life of around 3 days.
MK-7 (menaquinone-7) is the form of vitamin K2 with the longest half-life — about 72 hours, vs ~6 hours for MK-4. Practically: one dose of MK-7 keeps blood K2 elevated for 2-3 days; MK-4 needs to be taken multiple times a day to do the same job.
Most credible D3+K2 combo supplements use MK-7 for this reason. Look for "menaquinone-7" or "MK-7" specifically — generic "vitamin K2" can be either form, and the cheaper MK-4 versions are usually a worse value despite the lower per-mcg price.