Will these subwoofer be enough ?

While I have no direct experience with REL or Rythmic subs, I do with Hsu. I can confirm Hsu makes fine subwoofers that represent one of the best value and performance offerings on the market. I have a couple of his older column subs and they continue to serve me well. I can also recommend SVS as a source for good quality subs. SVS has a phone application that includes parametric eq so the subs in room response can be matched to the rest of the system. I can't say enough good things about this app and how much less effort it takes to get a reasonable blend between subs and mains. With a 4k budget you could get a pair of the their newest subs (15") in sealed box form, and for the size driver, the enclosures are relatively small. A pair of these in vented configuration would be noticeably larger and provide more output but would put you over budget. Nice thing is SVS gives you a free trial period to see if you like them or not... and if not, they are returnable for full refund. in the marketplace as it exists today, there isn't a much better deal to be found that I have run across. 

I like the overall sound of Khorns, though I have no recent experience with the version that is made today, back in the 80’s I knew a guy that had a pair of the then current model driven by a Harmon Kardon Citation 16 power amp and a Soundcraftsmen preamp that truly was a mindblowing system. He could crank that up and thru the closed doors and windows you could hear it a 1/2 mile away over top the traffic noise from a busy 4 lane highway. I have no clue what the actual decibel output was but it was up there. At anything like reasonable levels it sounded wonderful. My only real complaints boil down to a lack of very low bass extension, especially for their size.  They really would not work in my home, at least the way it is currently arranged, but it would be fun to live with a pair for a while just to see how it would be to do so. 

I'd look for a pair of used REL s812 - they're running around $1800-$1900 each and are better and will go deeper and are very well reviewed. And yes, I'd get two of whichever you end up with, a pair make more output, cancel nulls in the room and you can run each as a separate Left/Right. 

It really depends on what you are trying to do. If you are trying to accurately reproduce music. Then the subs you are looking at will do that. If you are looking to pressurize the room and overemphasize the lower bass, then larger subwoofers may be required. 

When I visit folks systems the first thing I typically notice is that the cross overs to the subs is too high and they are way too  loud, creating a huge bump in the bass... instead of an even transition and flat response. If you turn the subs up too loud, they will scramble the imaging. It's about what you are trying to do. I used to have two subs... they ran at less than 10% power and crossover of something like 40 hz. They extended the soundstage nicely. But it was so easy to turn them up too loud. 

@ghdprentice

I used to have two subs

Just interested in what you have now and why you changed?