You can leverage a policy action targeted to the 470 STS Relays to configure them to spread themselves across the 4 top level Relays using logic similar to what is described here:
Essentially, the Relays would be configured for manual selection (since Relays generally don't participate in autoselection), and the policy action will set the RelayServer 1 and 2 to different values based on their computer ID (which is generally random).
Here's an example actionscript (you'd have to update the Relay URLs in the actionscript below):
setting "__RelaySelect_Automatic"="0" on "{parameter "action issue date" of action}" for client
setting "__RelayServer1"="{item 1 of item 1 of (computer id mod 4, (0,"http://relay1.domain:52311/bfmirror/downloads"; 1,"http://relay2.domain:52311/bfmirror/downloads"; 2,"http://relay3.domain:52311/bfmirror/downloads"; 3,"http://relay4.domain:52311/bfmirror/downloads")) whose (item 0 of it = item 0 of item 1 of it)}" on "{parameter "action issue date" of action}" for client
setting "__RelayServer2"="{item 1 of item 1 of (computer id mod 4, (0,"http://relay2.domain:52311/bfmirror/downloads"; 1,"http://relay3.domain:52311/bfmirror/downloads"; 2,"http://relay4.domain:52311/bfmirror/downloads"; 3,"http://relay1.domain:52311/bfmirror/downloads")) whose (item 0 of it = item 0 of item 1 of it)}" on "{parameter "action issue date" of action}" for client
And here's a similar Fixlet on bigfix.me: https://bigfix.me/fixlet/details/27374 that you might adapt for this purpose.
Of course, you should test any of the above prior to deploying it in production or more broadly 