no expert but you need to understand more about the data transfer speeds between all the devices before you can say just changing the hard drive will fix your slow down problem. (which i seriously doubt anyway unless you change it to a ssd and not a hard drive.)
lets start from the hard drive, 4 folders each with 450MB of data (i'm assuming mb in your post is Mega Bytes and not Mega bits, 8 bits in 1 byte.) in each need to be transferred at the same time so that's 1800Mb of data.
now a hard drive has an average read speed of about 100MBs depending on things like where the data is stored on the platter and speed of hard drive.(ssd's are about 500MBs.)
next you need to take into account of how it's connected to the pc, if it is connected to your pc by usb 2 then the usb 2 connection is only capable of transferring data at about 40MBs so already in best case scenario it is going to take at least 45 seconds to transfer the 4 folders to the pc and then onto the 4 laptops (1800MB/40MB=45)
now if your hard drive is connected using the much faster usb3 type connection (about 400MBs speed) then this would cut the time down to 18 seconds because it would be able to transfer the data at the drives full 100MBs rate.
next you have the connection speed between the pc and the wifi router, lets assume wired so that's usually either 12.5MB(100Mbs) or 125MB(gigabit or 1000Mbs) so if your using the much slower 12.5MB connection then there is going to be slowdown here so your 18 seconds now goes up to 144 seconds, if it uses the 125MB connection then the time stays at 18 seconds.
ok from the wifi router to the laptops if wired then time taken is going to be either 18 or 144 seconds but if they are connected by wifi then the time taken will depend on what protocol they are using, lets assume 802.11n with 50MBs(400Mbs) now that 50MBs needs to be shared 4 ways so again that's 12.5MBs to each laptop which takes the time back up from 18 seconds to 36 seconds before the laptop gets it's needed 450Mb of data.
it might be your best option is to get a new pc that uses usb3.1 along with an external ssd drive to hold the data, or it might be your best option is to buy a better router so you can connect all 4 laptops and the pc at faster speeds like i said i'm no expert and that's just a rough idea of the things you'll need to look at before you can optimise your setup so minimize any slowdowns when all 4 laptops are accessing 4 different folders at the same time but i hope it helps you work out what is best for your situation.