I came across some statistics , that only about 4 % of dissatisfied customers complain, the other 96% just leave. So how do deal with these 4%

The ability to deal with problems as they arise is a key ingredient to success. Also, the organization needs to consider complaints as a gift! Why?

Customers who have an issue dealt with to their satisfaction have a 95% likelihood of repurchasing and telling 5 people about their experience; if they don't complain (as 96% of people do) they will tell at least 10 other people about their problem.
The occurrence of problems can cause a 15-to-30-point drop in high-satisfaction responses and in loyalty indicators. This puts revenue at risk to the average tune of 11%.

Dissatisfied customers by word of mouth will tell eight to sixteen others about their dissatisfaction. With the web some are now telling thousands. 91% of dissatisfied customers never purchase goods or services from the company again. A prompt effort to resolve a dissatisfied customer's issue will result in
about 85% of them as repeat customers
Depending upon the business, new customer sales may cost 4 to 100 times that of a sale to an existing customer.