I also tried using lineInit as per below but that did not work:

function lineInit(type, name){

if ((type == "item") && (name == 'custcol4')) {

var usefx = nlapiGetCurrentLineItemValue('item','custcol4');

if (usefx == "T"){

console.log("true");

nlapiSetLineItemDisabled('item','custcol5',false);

}

else {

nlapiSetLineItemDisabled('item','custcol5',true);

}

}

}

go through the documentation for Client Event Types

Also, it's good practice to change IDs on your fields so that you know what they actually are (or the poor dev that looks at your code later on....)

Using custcol* is kind of against suitescript since this might change in the future without any prior notice. If its not bfn certified code you can use vanilla js to achieve this