for (const i of [1, 2, 3, 4]) console.log('feature', i, process.env[`FEATURE_${i}`]);
console.log('feature1', process.env.FEATURE_1);
console.log('feature2', process.env.FEATURE_2);
console.log('feature3', process.env.FEATURE_3);
console.log('feature4', process.env.FEATURE_4);

feature 1 undefined
feature 2 undefined
feature 3 undefined
feature 4 undefined
feature1 undefined
feature2 test
feature3 undefined
feature4 false