yeast rafts take on many forms. when i use us04 i get floaty tan/orange colored clumps of yeast at multiple levels within the wort. some topside. some below. it's normal. yeast sometimes link hands and sing kumbaya for reasons only yeast know.
sour producing infections look like shrike's pics. when those snotty white clumps join up they form a 'pellicle'. it's like a semi-hard protective fingernail like scale to seal off the wort below and allow the bacteria to eat all your lovely alcohol. i had a lacto bacillus infection in a pumpkin weis that looked like pic 2. big snotty white/grey bubbles. there is a difference between 'sour' and 'tart' that many people get confused on. fermenting too hot produces a cidery green apple tartness that some perceive as 'sour'. true sour is more like eating unflavored, unsweetened yogurt. sour. puckering mouth, pinchy face sour. atomic warhead candy sour. sucking on lemons sour. tart like you get in wheat beers, is more like semi ripe cherry sharpness of flavor. or perhaps real cranberry in nature.
acetobacter infections produce vinegar flavors. the more alcohol the bug converts to acetic acid, the more intense the off taste until it gets so strong you can actually damage your mouth.
brett-c infections produce the sour similar to a lacto infection. people intentionally brew with brett c to make various sour beers that i am not a fan of myself.
infections arent that common if you exercise even the slightest good hygiene when brewing. there's an ancient thread here somewhere about Mashani's cat butt ale. lol... it's a classic.
A standard pack of Safale S-04 is 11.5 g, so it sounds like the supplier of your kit accommodated the smaller batch size and included just enough yeast for a 1-gallon recipe.
Be sure to reread @BDawg62's post above. Install a blow-off tube, place the fermenter in a tub and let 'er rip. Active fermentation with S-04 is more than likely going to fill that headspace quickly and then some.