water will dilute the concentration of hormone in your wee - that's why most HPT's suggest that you use first morning uring as it is usually the most concentrated (unless you're like me and drink all night too!)

at 4 weeks (i'm assuming you're 14days post ovulation) it's not unusual to have a faint line - as long as it's visible at arms length, it's a good sign. your GP should do a series of tests over a week (i'd not wait a week between tests!) to see that your hcg is climbing at a reasonable rate (doubling at least every 48 hours)

hope that helps

BG