what HD should i get?

I'm a little confused, since the RN102 model in your post has only two bays, and you are asking about four disks.  Did you get the model wrong?  Or did you mean two 4TB drives?

I suggest WDC Reds or Seagate Ironwolf for most applications.  I use WDC Reds in my own NAS.  Both are designed with RAID features, and both are quiet/cool.  Two TB models: WD20EFRX and ST2000VN004.  Four TB models: WD40EFRX and ST4000VN008.

Enterprise class drives are more expensive (and run hotter because they use more power), but offer higher performance and have longer warranties.  WDC Red Pro and Ironwolf Pro are two options here (there are plenty more).  If you actually have an RN100 series, your NAS isn't fast enough to take advantage of the extra performance - so I'd pass on these in that case.

I don't recommend desktop drives.  NAS-purposed drives are similar in price, and are better choices for RAID.