I don't use photoshop, I use brand X, but I think the general principles should apply. You need to know the scale the decals are now, and ratio that scale to 400. If you are making them bigger, the ratio needs to be a fraction bigger than 1, if you are scaling down, arrange the fraction so it is less than 1.
Then, scale by physical size option. Make sure you have enough pixels per inch. If you are enlarging, that is where you will lose ppi, so in the sizing you may need to select a resampling method. Scaling down is easier, because it actually increases ppi.
In most graphics programs you can select sizing change by percent- multiply the ratio I mentioned in the first paragraph by 100. If you do not have a percent option, you must calculate the final size by multiplying the original size by the scaling ratio.