best power conditioners under 4k

Unsure about 'best' option, but the Decware ZLC works well, is about 1K shipped and returnable.  Spending copious quantities does not ensure results.

The Inakustik AC-3500p has done an amazing job in my system. The difference between it "being in the system" vs "not being in the system" is significant. For me it is a worthwhile investment and the price is less than the budget you have set.
Check out the thread on this power conditioner, here on Audiogon. I was a skeptic, but am a believer now.

Richard Gray has been around for decades with various reviews.. I honestly never gave them a second thought as I tried and was generally satisfied with quite a few different brands over the years. Fast forward... a friend of mine who’s ears I trust recently bought a Richard Gray SubStation and raved about the increase in dynamics and the jet-black quieter background/ lower noise floor. So he loaned it to me for a weekend trial and the first five seconds sold me... It was everything he said and more.,.and I had to have my own. It’s not leaving my system and all my “audio buddies” familiar with my system agree. I’ve had it several months and as far as I can tell there are no negatives. Not particularly inexpensive but considering its transformational abilities, worth every penny of it. Good luck. 

An Isotek dealer in my area has a dedicated main power panel for the audio system function, including multiple isolated lines running into each demonstration room.  Even then, with a noise meter plugged into the power, there was plenty of noise on the line.  When the meter was plugged into the Isotek gear, there was either MUCH less noise or no detectable noise.  The audition of music also showed there was a substantial improvement in sound even if there was no actual "noise" that could be detected from the listening position without the conditioning--the sound seemed less artificial and more relaxed (without sounding dull or lifeless) and the soundstage more realistic (soloist seemed to be in a natural space rather than pasted to a spot).