Aw mama, ang layo talaga ng difference mo, four weeks is quite a gap! The heartbeat being strong at 164 is a good sign naman, but honestly kailangan mo talaga to get clarification from your OB on this one kasi it could mean a few things, either ang dating ng period mo ay off, or there's something with the measurement, and your doctor lang makakapag-explain ng tama at magbigay ng peace of mind. A few of us moms had similar scares and it turned out the dating was just miscalculated, pero better safe than sorry talaga, so book na with your doctor if you haven't followed up yet, ha?