just found this-

0-1 week: 0-50 IU/L
1-2 weeks: 40 - 300
3-4: 500 - 6,000
1-2 months: 5,000 - 200,000
2-3 months: 10,000 - 100,000
2nd trimester: 3,000 - 50,000
3rd trimester: 1,000 - 50,000
Non-pregnant females: < 5.0
Postmenopausal: < 9.5

dont know if it helps, but if implantion did happen 4 days ago then its highly likely its just too early depending on the level of hcg that the test requires.
as far as i know the digi ones arent that sensitive and if u havent used fmu, even more so