The scenario you describe will never be smooth with RMS.
The way I see it, you have two options:
For this example, Item XXX with Price $10
A. Put all four on a work order with a deposit amount of, say $30 (pay $10 each for two + 1/2 down on the other two). Tender to save the work order and take the money. Immediately recall the just created order, tender and ship partial the two items being taken with to create an invoice. Like I said, not smooth. There may be some issue involving the 'override deposit amount' flag.
B. Two separate transactions. Sell the two, then create a work order for the other two. Again, not smooth.
Option C: accept the RMS 'all or nothing' deposits on back orders and make your choice: all or nothing?
In any case, not smooth.
Tom