I haven't heard anything about it, but I have a simple question: why would you *need* two extra platforms at Swindon?
Let's look at the evening peak (I assume the morning is similar). The typical timetable headway on the GWML is 3 minutes, giving a maximum theoretical 20tph (trains per hour) departing on the main lines out of Paddington in the evening peak. Four of those are Heathrow Express, meaning a maximum of 16tph to Reading.
Of the current 12tph in the hour 1700-1759 leaving Paddington on the main lines, three of those trains (the 1703, 1706 and 1733) turn left at Reading to head down the B&H towards Newbury, and three of those trains (the 1718, 1722 and 1749) turn right at Didcot to head towards Oxford, leaving just six heading for Swindon.
Assuming that all four additional train paths were used for trains towards Swindon, that's still only at most 10tph calling at Swindon in the evening peak. At least one of those (the current 1742) will have to use platform 3 to access Cheltenham, leaving at most 9tph for platform 4.
Moreover, as I understand it the plans for the post-IEP timetable include some trains running non-stop from Reading to Bristol Parkway (or possibly even Paddington to Bristol Parkway). Assuming that's at least 2tph, that leaves at most 7tph to actually call in platform 4, and it's quite possible that one or more of those could use platform 3 while an up train is using platform 1.
Is that really enough to justify building two extra platforms? I honestly don't know the answer, and the answer may indeed be that yes, it is justified. But given that many, many platforms get more than 7 trains an hour, it doesn't seem immediately obvious why Swindon should get more platforms.