At three to 1 we have:
0.7x3 + 0.3x(-1)=1.8 meaning you will profit 1.8 dollars for each dollar bet in the long run.
At four to 1 we have:
0.7x4+0.3x(-1)=2.5 meaning you will profit 2.5 dollars for each dollar bet in the long run.
So you would have the mathematical edge in both of these situations for the odds you provided and the probability (70%) you provided.