Pressure is determined by desired level of carbonation and temperature. I have mine at 15psi/45°F. This would be way too high for 4 feet of line. I think you should replace the line with minimum 8 feet, if it's too much you can trim it down.
The chart below will help you determine the pressure needed. Calculators exist to help with line length as well.
![](http://www.drinktanks.com/wp-content/uploads/2015/02/CARBONATION_CHART_DRINKTANKS.png)