For almost 4 years I have been thinking about the same man, in ways that I have never experienced. It all began the first time a saw him, we did not talk but from then on I knew I wanted to get to know him better. Once we had actually met through a mutual friend, we seemed to click, but the draw back was that the two of us were in long term relationships both with children to consider.

Due to the fact that the friend in question had asked him to be his best man and myself to be the chief bridesmaid at their wedding we built up a friendship of sort, and eventually it became clear that we were both equally attracted to one and other, but there was still the question of each others partners. Neither of us were happy in our relationships and were together only for the childrens sake so the inevitable happened and the relation ships broke down.

I can clearly state that my feelings for the other person had nothing to do with the breakdown of my relationship, in that it had disintegrated long before hand. The other man and myself hardly see each other and if so only through special occasions or at weekends when we might bump into one and other, but each time i see him i feel an insatiable need for him so much that one night a few months ago we ended up having sex. It was completely out of the blue and exciting but nothing like I imagined it would be. In the morning things took on a completely new light when travelling back home he made no attempt to make speak or look at me which tore through my heart like a dagger.

Sounds like he was going through a guilt trip, where all that could go through his mind was he made a mistake having sex with you.  When men don't speak and don't make eye contact after sex, that is usually because they are going through a guilt trip.  By guilt trip I mean "I shouldn't have had sex with her, but I just couldn't stop myself".

I got his number from the friend and text him asking if everything was ok, which he responded to with yes. The week later i saw him and we ended up kissing, but later on in the evening he was chasing an other girl in full view of me this devastated me.

This obviously means he doesn't value you enough, even if he does fancy you.

I wanted to know how I stood so i text him again, this time i asked why he did such a thing and that it hurt me.

That would give the guy the creeps, even if he did fancy you, he doesn't want someone he is not having a relationship with yet, to be giving him a lecture.

He replied that he and his wife had just split up and he did not want commitment which i too felt.

Yes I agree, that after having a long term relationship, the last thing you want is to jump into another relationship.  But if this guy really fancied you all these years then he would have jumped at the chance of being with you.

I responded with this and that was the last I have heard from him.

You are very unlikely to get a reply, because he won't want to make the confrontation worse. 

I dont know what to do now cause this weekend our friends are planning a night out and i know that the two of us will be there too.

I would say go out and pretend he is not there.  Make no effort to see him and make no effort to contact him in any way.  What might happen is he gets drunk and is desperate for sex and then latches onto you again and wins you over.  You don't want to do that because he has hurt you once and is likely to hurt you again if you end up getting back with him.  Men should not be given second chances unless you have been together for a long time.  Which in this case you haven't been together for a long time.

I cant help feeling that he used me for some cheap easy drunken sex,

By what you said, it sounds as though he did.

but at the same time I cannot stop thinking about him first thing in the morning and when i go to bed and every minute in between,

You need to realise this guy has treat you badly and is not going to have any long term potential.  It is time to move on, there are plenty of other men, there's no point in dwelling on one idiot, find someone else.

when i see him my heart flutters and I feel sick i go week ant the knees and i feel the blood rush through my veins.

It takes a long time to get over people you have fancied or have been close too.  But believe me, you will eventually find someone far better, if you make the effort.

I know that i've used every cliché in the book but its all so true. I still want him after all this what is the matter with me especially after four years of all this, should I tell him how I feel

No definitely not, if he doesn't like you he will try his best to avoid you, because men don't particularly like emotional stuff.  If he does like you, which would be unusual by the way he has treated you and you start blurting out emotional stuff, you will more than likely give him the creeps and he will be off.  To me I don't understand why you are even considering being with this guy by the way he has treated you.

or will this add to his ego trip and make myself look even more foolish, if I dont how will I know how he really feels and if all the heartache is worth at at all.

Definitely not worth it, find someone else and yes his ego will be boosted and he will be calling you a stalker to his friends if you start trying to get back with him.