The laptop's motherboard will automatically run it in turbo boost. Depending on what is needed, it'll be variable cpu speed. Which sometimes goes up, or down. Check this link for a better explanation.
https://www.argotronic.com/en/turboboost.php .
You might want to ad a bit of memory though. Make that 8gb. That way, you'll be able to have more instruments open, if you resize a clip and don't delete the excess sound, etc.
Memory costs very little. Check if you can upgrade at the shop you buying the laptop or you can check online for the cheapest price. Make sure you know if there's available slots. Cas timings on memory differ. See if you can find out what is in it atm. Best , if you don't know computers too much, is to see if they can offer you a deal at the shop. I'd say an extra 30-40 bucks for 4 gig extra. Could be that there's 2 slots in the laptop and they used 2x2gb. Sometimes you can get the shop to resell those and give you an extra 4gb instead of getting the 2x2 and paying for 8gigs. Leaving with you with 4 gigs you're not gonna use.
I have 8 gigs on my laptop and 16 in my desktop. If i'd be using the laptop as heavily as the desktop, I'd also make that 16 gig. You can delete unused sounds and minimize instruments and tracks to save up mem and 8 gig will get you quite a distance.
4 gigs is kind of on the edge tbh.
The rest of the laptop will definitely be fine to run reason.
128 gb of hd space is more then enough for reason and windows. Though, I don't know what else you want to store. If you want to store a ton of refills, movies, series and tracks you've made, you might want to look into an external data disk. If there's an extra slot in the laptop for a disk, you could get an extra internal disk, instead of the external. Should be able to get TB disk for about 50 to 60 bucks. I always check the leaderboard of
www.storagereview.com and buy one of the HDs they mention there, according to what I'm gonna use it for.
SSD for my OS, DAW, (somewhere between 128 to 256gb) 7200 rpm or hybrid disk for data storage (500 gb to 1TB, depending on your needs). You could also choose to store on a cheap usb stick or disk, for, let's say, 20 or 30 bucks. Speeds can be slower, maybe less reliable.