You do not use SRTM v4.1 directly.
You use terrains, which are based on this data.
This is not the same. I have not seen any restrictions on using this in commercial products.

But if you are really worried about this, just use another source of elevation data:
Bing Maps, Mapbox or SRTM30.

I 'm reading user guide (Real World Terrain 3.5) p.4
"Commercial use
You can use terrains, buildings, roads, rivers, trees, grass, created with Real World Terrain, for
commercial purposes."

But, SRTM v4.1 is not permitted to use for commerical without permission.
(SRTM 3 is Public Domein, but SRTM v4.1 is actually "CGIAR's DEM data" and license is not Public Domein.)

http://srtm.csi.cgiar.org/SRTM_FAQ.asp#1
"If interested in using this data for commercial purposes please email Andy Jarvis. "

I want to use terrain for non profit purpose,(to make free game),
Would I use these data source of terrain?
or SRTM 3 option will be added?