Still works for me with the four items. Are you separating them with commas? If I didn't use commas with the four, then I got a blank page. I tried a couple of other four-item searches with commas, and had no problems.
In fact, it seems any search for two or more terms must be separated by commas.
I rebuilt the search index in an attempt to fix this error - but I'm assuming it's just the fact that vB search is far from perfect. A search for a combination of popular keywords like "high current pwm voltage" may just overrun the search. When in doubt, use the search bar at the top - it's powered by Google.