Easy lay down? Not really...
BTN has to call $18 to win $32.25, so we're getting pot
odds of 1.79:1, which corresponds to ~36%
equity required.
35.8% { 99 }
64.2% { QQ+, AKs, AKo }
33.2% { 99 }
66.8% { JJ+, AKs, AKo }
31.2% { 99 }
68.8% { TT+, AKs, AKo }
So yeah, its pretty close to a call, depending on how many pairs bigger than yours they stack.
However, if you're 4-betting 99, its because villain 3-bets a ton. And if they're 3-betting a ton of
hands, you should probably be calling off the 5-bet. This is a solved spot preflop, so I'll refer you to this handy chart, which is available in the Hold'em Manager article "Facing a preflop 3-bet". If you haven't downloaded the trial of Hold'em Manager and read that article, you're seriously missing out.
Opponent's 3-bet Percentage Required to 4-bet/call off preflop for 100bb's.
* KK+: Good against any range
* QQ: vs 3% or higher
* JJ: vs 6.5% or higher
* TT: vs 8.5% or higher
* 99: vs 10.5% or higher
* 88: vs 12.5% or higher
* 77: vs 14% or higher
* AKs: vs 3% or higher
* AQs: vs 8% or higher
* AJs: vs 13% or higher
* AKo: vs 5% or higher
* AQo: vs 9% or higher
This chart makes it so you never even have to know what your opponent's 5-bet range is (because its really hard to figure that out). And you'll never make a super exploitable mistake by folding to a 5-bet with a hand that has decent equity.
Also this is why most people 4-bet smaller (2.5x). Because once you 4-bet to a pot sized amount, you're usually getting close
pot odds to call it off with whatever hand you have, and 4-bet/folding becomes a mistake.