JSFiddle - React, Tailwind, and code Playground

JavaScript

var a = [11, 2, 3, 4],
    b = [2,  6, 3, 5],
    c = [4, 3, 20, 3],
    d = [34, 2, 21, 5];

function findCommon()
{
    var bucket = [];
    var maxVal = 0;
    var minVal = 0;

    	for(var i = 0; i < arguments.length; i++)
        {
            for(var j = 0 ; j < arguments[i].length ; j++)
            {
                var val = arguments[i][j];
                bucket[val] = bucket[val] + 1 || 1;
                if(val > maxVal)
                {
                    maxVal = val;
                }
                else if(val < minVal)
                {
                    minVal = val;
                }
            }
	    }

    var retVal = 0;
    var maxTimes = 0;
    for(var i = minVal ; i <= maxVal ; i++)
    {
        var val = bucket[i];
        if(val > maxTimes)
        {
            maxTimes = val;
            retVal = i;
        }
    }
    
    return retVal;
}

console.log(findCommon(a,b,c,d));