Hi Argothair,
I like to have more tanks so they can work as a group and help defending India, China and Key Russia territories at the same time when needed. 4 is a bit too little to me and 6-8 makes a decent number to strike back. I like the mobility of tanks as well as its better attack/defense power. Also, in round 2, Germany has less chance to stack the Karelia if I have 6 tanks available compared to 4 tanks.