If you mean combination of digits then there are 8×7×6×5/(4×3×2×1)=70 combinations.
If you mean permutations (that is, how many 4-digit numbers there are formed from the digits 1 to 8) then we need to multiply the number of combinations by the number of different ways of arranging 4 digits=1×2×3×4=24, and this gives 8×7×6×5=1680 different numbers.