AMT shafts increase in 3 grams increments- soft stepping a 4 iron (once) would be inserting a 3 gram lighter 3 iron shaft- an approximate 1/3 decrease in swingweight. Cutting 1/2 inch to 4 iron length will further lighten the shaft but I don't know by how much. In any event the final swing weight change (after cutting to 4 iron length) shouldn't be significant.

Each 1/2 increase in shaft length equals 3 swing weight points so you could cut longer than 4 iron standard length to offset- one quarter inch increase would seem reasonable with slight increase in playing lie angle. I am not an expert, but, hopes this helps in your decision.