Good god just got out of the way of em stampeded horses, there must be Indians around these parts !!!!
If there is any chance of the stack blocking & the removed connection being put under pressure I would want to be using something that is going to stay in place & remain water tight.
Either a 100mm Timesaver or Ensign coupling (GT01) with a Blank end (GT70) cap, is the proper way to do it, IMHO.
At a push I am with Kozak above, they do a proper internal plug but you only need about 25mm of plan pipe showing to get the coupling on the outside.
[DLMURL]http://www.saint-gobain-pam.co.uk/assets/docs/sdr/Timesaver_Manual_2012.pdf[/DLMURL]