Question about Synology DS124

Your question is related to a single disk device, the DS124 can't have more than one disk at any given moment.
If you choose a diskstation supporting 4 disks, you can insert different size disks, but if you want a single volume the total volume will be based on the smallest size disk, the others will only be used at that same capacity. So if you have 2x 4TB and 2x 8TB, your total space will be based on 4x 4TB. You can, however, allocate two separate volumes of 2 disks, one of 2x 4TB and one of 2x 8TB. This may not be what you desire but it is possible. Based on a single volume as stated, if you replace the 2x 4TB at a later date with 8 TB disks, the diskstation will automatically adjust to maximise the volume size to 4x 8TB.