First, let's find a common denominator for the fractions:

3/10 + 4/15 = (3/10)(3/3) + (4/15)(2/2) = 9/30 + 8/30 = 17/30

-3/10 - 4/15 = (-3/10)(3/3) - (4/15)(2/2) = -9/30 - 8/30 = -17/30

3/10 - 4/15 = (3/10)(3/3) - (4/15)(2/2) = 9/30 - 8/30 = 1/30

-3/10 + 4/15 = (-3/10)(3/3) + (4/15)(2/2) = -9/30 + 8/30 = -1/30

Therefore, the answers are:
3/10 + 4/15 = 17/30
-3/10 - 4/15 = -17/30
3/10 - 4/15 = 1/30
-3/10 + 4/15 = -1/30