With the WCL - the experiment actually uses up consumables - the water, the various pellets that get dropped in - at the end of it you've got mars soup + additives in the beaker. Even if you drained it, you would have contamination remaining. 4 is just the number they could cram into the 'box'.

Tega - they're having enough difficulty getting soil into the tiny ovens - getting it out again would be near impossible.

But the major reason they've not spent the time and money and volume and mass and complexity to make them reusable? They don't need to

We're 70 sols into a 90 sol mission, and we've used half the WCL and a third of the TEGA shots.

Doug