Single mom of 4yr old twin boys--1 defiant, 1 not?

One of m.y boys is so defiant recently. I feel like all I do is yell at him & argue with him. The other twin is well behaved & is a big helper, but ends up being "ignored" as I spend so much time with his brother. So ultimately I feel like a complete failure. PLEASE HELP!!!!!! They have no one that disciplines them besides me. I am all they have and I feel so overwhelmed. ANY suggestions are welcome. I love my boys more than anything and want to be the best Mommy to them...