I don’t know its right or not to show the way to Foursquare. But what I have investigated that Foursquare creates thumbnails of sizes 30 x 30 and 110 x 110 etc.
I formed the image path like this
for small image
user->photo->prefix.'30x30'.user->photo->suffix;
for larger size
user->photo->prefix.'110x110'.user->photo->suffix;
That is I have concatenated them with (30×30) and (110×110).
Thus the above image becomes
https://irs3.4sqi.net/img/user/30×30/K4VCI4MXHWFUGXOF.jpg
and
https://irs3.4sqi.net/img/user/110×110/K4VCI4MXHWFUGXOF.jpg which are valid till now.
Hope this will work until they change this.