first, u should know in chinese there are 4 kind of words can 重叠, verb,adjective,noun and quantifiers. sometimes 匆忙and 急忙are adjectives and sometimes they can be advers too,so when they are adjectives can 重叠.but 连忙and 赶忙are adverbs just,so they can't 重叠. when 匆忙and急忙 are adjectives,they are diffrence from 连忙and赶忙.匆忙和急忙except can be used to describe people's hurry action and also to people's emotion.eg:你怎么这么匆匆忙忙（or 急急忙忙）的，时间还早呢. but for 连忙和赶忙，we can't put them in this scentence,they just be used to bedeck verb.eg:趁明天之前连忙（赶忙）把作业做完. hope these can help u.