What is meant these four words?

It is a line in the some song. They are sung inscribe:
"hirari, guririn, hararin, fuwari"

They are all onomatopoeic words - words that depict and mimic movements or conditions. "Guririn" is a bit unusual, but all others describe something lightweight, like feathers or flower petals, falling . "Guririn" adds a bit of speed, as it something is falling very fast in a circular motion.

I agree with AK about the other words, but I'm not sure if guririn is even an onomatope. It's a character in the manga "Hagane no Renkinjutsushi".

http://dic.pixiv.net/a/%E3%82%B0%E3%83%AA%E3%83%AA%E3%83%B3

gur[i]rin is from guri-guri-to[グリグリと] (no guri-gurin)
hararin also from harari[はらり] and hara-hara-to[はらはらと]
if different word "gur[u]rin" from gururi-to[ぐるりと].

+nn[ん] style is not unusual make two different meanings
one is motion moving/action like "-ing"
but not valid for all onomatope words just +nn matched words not a lot.
ex)
pikarin from pikari or pika-pika, kirarin[きらりん] from kirari or kira-kira
sometime more meaning stronger "-っ![-'!]" plus as one word like kirarin'! [きらりんっ！]

Other one is be soft/cute/friendly/child-like as instead "-san/-chan".
That anime/manga chara(also real human/pet/tools) names are so.
Girls name nick not only -nn "-rin/-chin/-tan" instead a lot.
ex)
Ryota(boy name) is "Ryota-n", Sho(boy name) is "Sho-tan/-chin"
Misa(girl name) is "Misa-rin/-chin/-tan"