It depends on what kind of a Buckeye tree you planted, if its a bottle nose one I don't see where there would be any problem. Here's a picture of mine.
It will never get any bigger then that.
It's never a good idea to plant a tree close to any kind of drain lines. But I really don't think you'll have any problems because it's a solid pipe and you are 4 foot away from it. I would think the roots would end up growing under it far enough below ground not to cause a problem. I have seen maple tree roots grow large enough and come up close enough out of the ground that you would have to worry about the line being lifted up by them even at four feet away. A good rule of thumb about tree roots is to figure the root system below ground is about the same as the branching system above ground except for pine trees they don't have much of a root system and are more likely to get blown down in storms with high winds, but even pines send down one main tap root to help anchor the tree to the ground. First find out what kind of Buckeye tree you have then look up specific details about that kind of Buckeye to find out how big it will grow and how much you can expect the root system to grow over the life of the tree. Right now with the tree being small 4 feet away from the septic line won't cause any problems. But you have to think about how big is that tree going to be 10-20 years down the road? Thirty-one years ago my husband and I bought a six acre field and every year planted more and different kinds of trees. We felt at the time they were spaced out nice. But after 31 years of growth most of them are really crowded and really big now. I have a lot of drain lines in my yard that I wouldn't want to have to dig up now that the trees are big, but on the other hand I've never had a problem with the roots messing up the drain lines. The roots are either above of below them, I don't know, but never had a problem with them ( knock on wood ) LOL...
You might consider calling your county extension agent and asking them for their opinion. They can be a great help with explaining the properties of your soil. I would think the type of soil you have would also play a big part on how your drains and tree roots function in your type of soil.
