It depends on whether or not you are saying that you've got a 4G drive that is entirely allocated to be the /ftp directory, or if you've got a drive with multiple partitions, one of which is 4G in size and is being allocated to /ftp.
If it's the former, then No, there shouldn't be any fstab changes necessary because you are merely replacing the existing hard drive with one with more capacity. All other things, including the device and partition names would remain the same, and thus, your existing entry for /ftp in the fstab file will remain valid. To illustrate, if your /ftp directory existed on a 4G drive that was located in the primary slave position, then your ftsab would contain a line roughly similar to the following
/dev/hdb1 /ftp ext3 defaults 1 2
If it's the latter case however, then what you really are doing is relocating the /ftp directory from one device/partition to another. In this scenario, your existing fstab would need to be updated, in order to correctly specify which partition on which physical device the /ftp directory now exists on. To illustrate, if your existing /ftp directory corresponded to the third partition of your primary slave (ie, /dev/hdb3) and with this new drive you were going to allocate all 100G as a single partition and install the drive in the secondary slave position (ie, /dev/hdd1) then naturally you would need to modify your /ftp entry in fstab to now start using /dev/hdd1 instead of the existing /dev/hdb3.
To illustrate this concept, the "before and after" versions of the /ftp entry would look something like this
/dev/hdb3 /ftp ext3 defaults 1 2
/dev/hdd1 /ftp ext3 defaults 1 2
Again, because /ftp now resides on a different device/partition, the fstab entry would need to be updated.
Finally, at risk of telling you something you already know, for reference:
/dev/hda = primary master
/dev/hdb = primary slave
/dev/hdc = secondary master
/dev/hdd = secondary slave
Hopefully this makes sense. Good luck with it -- J.W.