K535W is a four grain motor, just like the K550W. In fact, the K535W is the single use version of the K550. Fly them side-by-side, expect the same results. If I remember correct, they are the same grains...both "standard" White Lightning, same length and core size.
Minor variations may exist...or may simply be a product of change over time: IE: the K550 was first certified/tested decades ago, whereas the K535 was first certified within the last few years. New people at Tripoli TMT, NAR S&T, different equipment, K550 single throat nozzle vs. medusa, etc.
Nevertheless, both 4 grain standard White Lighting motors. Again, fly them side by side and expect similar results given that the other variables are constant. Yes I've flown both; no I can't objectively tell the difference in flight profile and performance. Both great motors; which to me are functional equivalents.