For 4.5hrs? Are you sure about that? With lounge access and average passport check and immigration time, that would give him roughly 3hrs in the lounge, possibly a bit more. Not the end of the world. A few cold beers, some snacks (personally I found the food quality - when the time is right and there IS warm food served - to have been improved), a nap and checking the news. Just average, but certainly could be worse.

If you don't hold a visa, your 4.5hrs will most probably be cut by quite some lengthy queue for TWOV, and they might (quite rightfully!) deny it eventually due to the short stay - unless you actually checked a bag. Anyway, even if we assume just 1hr to actually GET the TWOV and pass immigration AND take the train to the main area AND find your luggage, that will mean you've just about 2hrs to spend. Which will include a bag with you in tow (ok, you can head up to departures, check the bag, then have about 1.5hrs without your bag when you get downstairs again)

With a Visa AND without lounge access, the situation might be a bit better (even though I'd probably prefer to skip those two stamps) but not enough to change anything in the situation. Maybe heading landside indeed then. But I just assume that OP does either not have a (unlimited) Chinese Visa or isn't lacking lounge access. And then it would be a no-brainer for me to stay airside.

Sure, the bar in the Hilton is decent, but in all honesty, that would be quite a treck when all you could do instead would be a quick passport check, heading down to security and off to the lounge you are.

In my opinion, the Hilton is great if you can make use of a decent rate + get lounge access/upgrade on a relatively long stay (say, 5pm arrival, 9am departure) that's too short to head into the city but long enough to use it's facilities. But if you got a day stay (up to around 5-6hrs, over which I'd probably consider heading for the city and have a meal there) I'd simply stay in the airport, airside. And for a very short night overlay (of which I've done quite a few, with one of my favorite flights departing at 6.45am and I usually arrive the night before at very late time) a much cheaper alternative like the Ibis will just do fine. If you arrive past midnight (or well, late..), and all you want are 4-5hrs of sleep, I'd prefer to spend 200 RMB for a reasonable bed over 1000 RMB. Or whatever the current rate of the Hilton/Langham are.

I personally LOVE the Hilton (Is Cindy still working there? Awesome staff!) Beijing Capital airport, but there are times (or better said, timings) when it's usage makes sense, and there are those when it doesn't. That's at least my opinion.