This chapter doesn't deal with Samuel himself, though he is mentioned briefly in the first verse. Reading over chapter 3 again it actually seems like the first part of that verse belongs more in the context of the end of chapter 3... but that is a good reminder that these chapter and verse distinctions didn't exist in the original documents.
The bulk of this chapter deals with the Israelites going to war, and bringing the Ark of the Covenant with them thinking it would grant them victory. I find it javascript:void(0)interesting that they did that, but we aren't told that they asked God for success in battle. That *almost* makes this border on idolatry in my mind: they thought the presence of God's dwelling among them (the Ark) would ensure victory, when it is God Himself would could do that - whether or not the Ark itself was present.
When they are defeated, despite its presence, the Ark is taken by their enemy - the Philistines. Word of that reaches Eli, who collapses and dies. It is interesting that we are told he had been concerned for the Ark's safety, but no mention is made of his being worried about his sons who were there with it.
From a military perspective, the battle described in this chapter is a pretty serious blow to Israel. The loss of 30,000+ troops in a single engagement is tremendous; to put that in perspective, it is three times the number of casualties the Allies had in the combined D-Day landings - and more losses than either side sustained in the battle of Gettysburg during the American Civil War. I'm not sure what size the combined Israelite army was at the time, but from the description in the text it was a massive defeat.
Why would God have permitted such devastation to be visited upon His people? I can't answer that in full, but I can say that the coming chapters will see God turning a heavy hand upon the Philistines for taking the Ark, and I'm sure that the judgement prophesied against Eli's sons - who died in the battle - played a part.