react.js every nth item add opening tag or closing tag

render() {
   const rows = array_chunk(this.props.columns, 4)
   return (
     {
       rows.map((row) => (
         <div className="row">
         {
           row.map((col) => (
             <div className="col">{ col }</div>
           ))
         }
         </div>
       ))
     }
   )
}

An example array_chunk (I recommend that you use lodash)

module.exports = function chunks(arr, size) {
  if (!Array.isArray(arr)) {
    throw new TypeError('Input should be Array');
  }

  if (typeof size !== 'number') {
    throw new TypeError('Size should be a Number');
  }

  var result = [];
  for (var i = 0; i < arr.length; i += size) {
    result.push(arr.slice(i, size + i));
  }

  return result;
};

I actually just used arrays and react handled fine the rendering.

render() {
    let rows = [],
        cols = []
    let index = 0
    const totalCols = 20;

    for (index; index < totalCols; index++) {
        cols.push(<div class="col" key={index}/>)
        if ((index + 1) % 4 == 0) {
            rows.push(
                <div class="row" key={index}>
                    {cols}
                </div>
            )
            cols = []
        }
    }
    return (
        <div class="container">
            {rows}
        </div>
    )
}