no,it shouldn't be 5W....for example if your input is 18V and battery is charging at 13V,then 18-13/500= 10mA current will flow through it.So,its power should be >0.05W..
i fear,you will damage your battery if you charge it at 4A... 4A is much greater for 6Ah battery....
anyway,0.3E resistor should be greater than 5W ( about 8W or 10W )...
yes,you can use it,,now equivalent will be 0.2E,So,now power rating for each resistor should be greater than 1.6W...you can also use 2 x 0.1E of each 5W rating
In the first circuit at this page what is the wattage of resistor named see text. I have set charging current to 600 mA. so 0.6/0.6 = 1E resistance. What should be wattage 1/4W, 1/2W, 1W, 5W ?