If your volume is using Synology Hybrid Raid (SHR) adding a drive is very easy. See this tutorial. That is what I did when I went from 3 to 4 HDD's in my DS411+ii. You can set up a SHR array with just one disk if that is not what you are currently using. Keep in mind with just one disk you will not have redundancy. With 2 4TB HDD's only 4TB will be available on the SHR array, but you will have redundancy. The volume will expand once you have 3 or more 4TB HDD's, but since you have a 5 bay device I imagine you planned on expanding anyway.
In response to your comment: If you do not want redundancy but do want the ability to expand the disks later, set up your volume as JBOD. According to the first tutorial in section 2: "DiskStation supports expanding capacity of volumes and Disk Groups with the following RAID types: Synology Hybrid RAID (SHR), JBOD, RAID 5, RAID 5+ Spare, and RAID 6." Thus, the only option which allows expandability without redundancy is JBOD. This tutorial will show you how to create a JBOD volume if yours is not currently one.