The most important competitor which Samsung Galaxy S4 will have right now is LG Optimus G, i will map the similarities and differences here, in this post. At first glance the two seems to resemble very much, having almost the same software and hardware. Both have to make a lot of noise if they want to stay competitors in the industry of mobile phones.
Also, both phones are well built, this Samsung phone resemble a lot with Galaxy S3 and has lower margins, and LG is a business phone because it has a classic design. LG is only available in black color, for Samsung we will choose between black, white, grey, blue, red. The buttons are missing from both, having only touchscreens, both will have 2GB of RAM, performant graphics cards, 13 MP camera. Samsung will have a 2 Ghz processor, a quad core most likely, while LG will only have 1.5 Ghz processor.
Both displays are extraordinary, made by different methods, though. The Samsung display is AMOLED, 5 inches, while the LG display has more than 16 million colors available.Samsung is more expensive but had a sales record last year, both running Android operating system.
My verdict: I personally think that Samsung is more beautiful and more powerful, but it is only my opinion.