
Brooklyn Bedding
Signature Hybrid
Durable, comfortable, adjustable firmness; excellent value.

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You should be able to get a quality mattress for 2k. I was able to get a kings sized Kingsdown for 1200, though that was after haggling. If you do your research you should be able to find something good.
I just bought a medium-firm top of the line Kingsdown spring mattress (2500ish dollars) and I couldn't be happier. It is the most comfortable mattress I've ever had. No off-gassing whatsoever and am very smell sensitive. I did a lot of research as well before settling with that one. No regrets so far.
So with the $700 one, it may have been a used model or possibly a discontinued model. I know with Kingsdown they are made to last 14-20 where as everything else is made to last 7-10 years. Kingsdown is also made with pressure relief in mind. 4k means you are probably looking at the W series at mattress warehouse. It’s got reinforced foam from head to toe (different levels of support) along with a luxury support band in the middle third of the bed for lifted lumbar relief. This isn’t to say the other beds are bad (I sleep on a series 2 medium pillow top but my mother sleeps on the medium Kingsdown in question), but Kingsdown is a cut above the rest. They have been around since 1904 and DO NOT ADVERTISE. What company do you know has successfully been around for over 100 years with no advertising? Because Mattress Warehouse has the Kingsdown, go there. Get a mattress protector with the bed and try it out. They have a comfort exchange. If after 30 days you get it, you get up to date 120 to do a switch. You get full access to what you spent the first time to go toward the second bed of equal or greater value (unlike mattress firm who takes 10-20%). So if you don’t LOVE the Kingsdown, you can upgrade to the series 3 Beautyrest or the sterns. Plus mattress warehouse gives you a price match guarantee. First year from purchase if the bed goes ANY cheaper during a big sale, you call and get a price match refund. Give them an email and you get an email every time there is a new sale. You just call and ask if it’s cheaper, when it is you get a refund. Kingsdown is also known for having amazing support with differing comfort feels. Same pizza, different topping. Their firm plush and euro top models all have the same support system with a different comfort feel. And they are also the only beds that are TESTED before leaving the factory. Every single bed goes through a machine which pressure tests it to the original spec beds that they update every now and again. If it doesn’t pass they doesn’t sell it.
I have a Kingsdown that's 10 years old and still going strong. It's a spring mattress with a thick pillow top and firm support underneath. I want to say the exterior fabric is bamboo but I'm not sure. I can say it sleeps pretty cool whatever fabric it is. As a side sleeper too, it's incredibly comfortable and I actually miss it when sleeping on a hotel mattress, so I'd say it's better than hotel quality. I have no idea what their current offerings are but if I was in the market, that's the first brand I'd look at. I recall it being a little pricy 10 years ago so I can only imagine what the price tag on a similar modern one would be though.
I've had my Kingsdown latex and foam mattress for at least 12 years. There is an indent where we sleep now, but it's still extremely comfortable. I bought it on a whim because it was on clearance and it's the best purchase I've ever made. It's insanely heavy and has no carry spots; it took 3 adults struggling to carry it when we moved. That's my only gripe!
COSTCO CANADA "KINGSDOWN" mattress. (k/s) Very supportive 2 years in. Don't know if Costco USA carries them.

Brooklyn Bedding
Signature Hybrid
Durable, comfortable, adjustable firmness; excellent value.

Sleep EZ
Organic Latex Mattress
Highly customizable layers, durable; but extremely heavy.

Helix
Midnight Luxe
Pain relief, side sleeper support; but some find too soft.

Purple
The Purple Mattress
Unique grid feel, cool; but heavy, expensive, polarizing.

Brooklyn Bedding
Aurora Luxe Cooling
Excellent cooling, zoned support for heavier sleepers.

Ranked #1
Novaform - Platinum 16.5" Luxury Hybrid Mattress

Ranked #1
Brooklyn Bedding - Titan Plus Luxe

Ranked #1
Purple - The Purple Mattress

Ranked #1
Sleep EZ - Organic Latex Mattress

Ranked #1
Helix - Midnight Luxe

Ranked #1
Sleep EZ - Organic Latex Mattress