You should mix 4µl of DNA sample and complete it with 96µl (100-4) of water.
You can figure this out by using C1.V1 = C2.V2
C1 = 250 ng/μl (Sample Concentration)
V1 = ? (the necessary volume to take from the DNA sample)
C2 = 10 ng/μl (PCR concentration)
V2 = 100 µl (total volume)
V1 = [tex] \frac{C2 . V2}{C1} [/tex] = [tex] \frac{10 . 100}{250} [/tex] = 4µl
For the water: 100µl (total volume) = 4µl + Water
Water volume = 96µl