Nothing in particular. Look for any check engine or ABS lights to stay on while riding. The dash lights cycle thru when key is turned on, but the ABS light normally stays on until moving on 2014-on bikes. Make sure they have the Red programming key as well as the normal use black key(s). Bikes come new with three keys, one red and two black. W/o the red key, you can't get new keys made. Ask if they have the third lock for the panniers/top box. Every bike comes with three key matched locks for panniers/top box. Two are currently installed in the panniers, but a third should come with the bike. Those tiny T-10 torx screws that hold the locks in tend to come loose. A Good Idea™ is to check them and put a single drop of locktite blue on the screws when re-installing them so they don't vibrate loose and leave you with a locked bag with the lock having fallen inside. Remember when opening the pannier lids, Press DOWN on the lid, then turn the key. This saves a lot of stress on the mechanism and makes it much easier to open the panniers. Check the rear brake pads closely. Because of the linked brakes, some riders very quickly run down the rear pads. (short trips with lots of light braking are harder on them than longer rides). Ask/check if the factory owner's manual is still with the bike. Some owner's log all service, so beyond the info, it's nice to have and you should have one to familiarize yourself with the new to you bike too.
If you see any scratches on the left side upper black panel, ask them to remove that panel so you can verify the condition of the radiator and fan. The fan can get pushed into the radiator and seize the blades, burning out the fan. It's expensive to replace the fan. The radiator usually only gets some minor damage to the fins.
Valve service/check is not due until ~42kms, so quite a ways off yet. As a 4 year old bike some of the fluids should have been changed or are due. Brake fluid should be clear to light amber and ask if they know if the coolant has been changed. I would suggest you offer to the seller that you're out of hand going to need to have the following maintenance items done, just to know that service is correctly up to date, (unless they can document the service history), and get pricing for those at your local/preferred dealer, then use this as a bargaining tool to negotiate the pice down. (All you can do yourself easily.)
Flush and bleed both brake and clutch systems & function test ABS system, (should be done every 2 years)
Flush coolant system (every two years)
Replace fork oil on both forks
Change final drive oil
Change the engine oil and filter
Check all spokes for proper torque (52 in/lbs) Check to see if wheels are true.
Replace the air filter (check at 7000 km, replace at 19000 km, but may require replacement earlier if not checked/cleaned)
Replace the spark plugs (due at 13000 km)
Sync Throttle bodies, (due every 6000 kms) (usually require a minor adjustment early on and stay fine for a long time after that. Typically checked at valve clearance check time after the first couple of syncs)
Re-pack the swing arm bearings (due at 13000 kms) No one does this until much longer.
Check wheel bearings (check due every 6000 km) Usually last 160,000 km, but can become damaged depending on rider and conditions.
Re-pack steering head bearings soon, (due at 16,000 km). You'll want to at least check this. They are often found to be loose early and only lightly greased from the factory.
Lube brake and clutch lever pivot shafts (due every 6000 km)
Remove, clean and lube shift lever (common issue with grit building up and making shifts notchy)
Brake lever pivot, side stand pivot and center stand pivots should all be lubed every 6000 km too. (rarely done, may or may not need attention)
Hope that info helps. They are a fun and trouble free bike that is easy to maintain and requires little attention if you do normal maintenance.