Well, here we can use the same trick that fstab uses for Optical media (aka CD's and DVD's):

/dev/sr0        /media/cdrom0   udf,iso9660 user,noauto     0       0

The first column indicates the file system, I'm sure your USB's will have fixed values, so lets presume that everything after sda is a USB, and since you can only have 4 USB's at the same time, the list goes from sdb, sdc, sdd and sde.

$ ls /dev/sd*

/dev/sda   /dev/sdb   /dev/sdb3  /dev/sdb6  /dev/sdc  /dev/sdf

/dev/sda1  /dev/sdb1  /dev/sdb4  /dev/sdb7  /dev/sdd

/dev/sda2  /dev/sdb2  /dev/sdb5  /dev/sdb8  /dev/sde

_{(In my case I use a memory reader, so my USB drivers start from sdg but lets continue.)}

Now, we assume each USB have one and only one partition, so the lines we need, for our fstab, are:

/dev/sdb1

/dev/sdc1

/dev/sdd1

/dev/sde1

Then you said your mount points has to be fixed, so after you have created your directories, lets add them:

/dev/sdb1 /media/HDD1

/dev/sdc1 /media/HDD2

/dev/sdd1 /media/HDD3

/dev/sde1 /media/HDD4

Since you can have almost all type of file systems, lets use the auto so fstab guess the file type to use:

/dev/sdb1 /media/HDD1 auto

/dev/sdc1 /media/HDD2 auto

/dev/sdd1 /media/HDD3 auto

/dev/sde1 /media/HDD4 auto

Ok, now lets boil down to the options, which will be the very same for CD's with the plus of read/write permissions:

/dev/sdb1 /media/HDD1 auto rw,users,noauto,allow_other,umask=0

/dev/sdc1 /media/HDD2 auto rw,users,noauto,allow_other,umask=0

/dev/sde1 /media/HDD3 auto rw,users,noauto,allow_other,umask=0

/dev/sde1 /media/HDD4 auto rw,users,noauto,allow_other,umask=0

rw tells that we want read and write permission, users allows any user to mount a device, noauto prevents that the driver gets mounted automatically when mount -a is called, like it does at boot, preventing boot ERRORS and WARNINGS, allow_other allows other but the user that mounted the driver to have the same permissions, and umask=0 prevents the defaults umask from being applied.

Now the only 2 left fields are pretty much just about the dump order and fsck order, which could be in 0 both, leaving us with the final result:

/dev/sdb1 /media/HDD1 auto rw,user,noauto,allow_other 0 0

/dev/sdc1 /media/HDD2 auto rw,user,noauto,allow_other 0 0

/dev/sdd1 /media/HDD3 auto rw,user,noauto,allow_other 0 0

/dev/sde1 /media/HDD4 auto rw,user,noauto,allow_other 0 0

With this, each time that you plug a USB device it will being automagically mounted in the mount point without the intervention of the user. You should modify according to your necessities.