You said there were 4 in your house and with your 3 brothers and you I am wondering where your parents are. They should be the ones to dole out consequences but if you are the one in charge and their are no parents then you have to take action.
First, just confronting the issue is not enough. Remember, this is an addiction and you caught them they did not come to you. Therefore you must have immediate and hard consequences. So hard that they will not want to see porn again.
It will be easier for you if your 18 year old brother is not also watching porn because you will have his support as well.
It will be hard for the two of you to put up with the consequences as well because it might involve just not having a computer for awhile. Their are companies that provide filtering for your computer too and when they finally have endured the period of time that you set up for no computer at all, then these protections need to be in place.
Second, I would suggest not allowing them to hang out at any time anymore with those friends that got them into it. Also, the parents of those kids that got them into it need to be informed as well. If they are decent people they will not want their kids involved in it either.
Third, try taking a few privileges away from the two younger brothers that they really really like. Weather it be hanging out with friends, nintendo, whatever, choose something that will really bring the message home to them that this type of behavior will not be accepted. You may want to make this period of time for this particular punishment shorter than the one on the ban from the computer.
Fourth, realize that even though you love them they cannot be trusted to not watch porn even at their friends house. Trust must be earned. You may say that this is the reason why they are grounded from going to their friends houses and later when they are allowed to hang out with their friends it will have to be done at your house under your supervision until trust is again built up between you.
As they start to prove trustworthy they begin to earn privileges back again. Privileges such as hanging out with friends, video games, computer use, and even rewards such as going somewhere that they really want to go, or getting something that they really want can be used as incentives to change their behavior.
Make sure that they understand what the consequences will be if they ever fall back into the destructive habit of watching porn.
All of the preaching in the world will not do much good at this point. They have to be taken out of the situation, and slowly they light of the gospel of Jesus Christ will start coming back into their eyes and into their hearts.
I know this sounds like tough love, and it is, but sometimes tough love is the best thing for the child. Always remember to do it with love not anger and hate.
My son when he was 12 was also introduced to Porn on the computer by a friend. My daughter caught it in the same way you did by looking at the history. We confronted him about it and he admitted it. We banned him from the use of the computer and grounded him for a month. It was summer and he wanted to hang out with his friends. He was a good kid and was ashamed of his behavior. He had to come up with excuses to tell his friends why he couldn't hang out. We kept his secret too. He didn't want anyone to know why he was grounded. He wouldn't tell us which friend got him into it but we guessed anyway. We advised him to not hang out with whomever it was anymore and he didn't.
He never got back into Porn again. He has served a successful mission now and is one of the best people I know. His friend completely dropped out of church activity and never served a mission. Their lives are completely different from one anothers now.
I am proud of you for wanting to nip this in the bud and help your brothers. It will change their lives. Watching porn is not a little thing, but a big one. Our thoughts control who we are and what decisions we make.
Please include the Lord in your decisions of what to do. He is always there to help and to comfort you. He will inspire your heart and mind to know what is the best way for you to solve the problem.
Good luck to you and God Bless.