All four words (reject, decline, refuse, spurn) indicate that the actor is saying "No" to something offered or requested. Of these, "decline" is the softest. It suggests that something was offered or requested but that the actor did not accept the offer or did not grant the request. "decline" can be used with words such as "politely" or "graciously" as in "the girl graciously declined the invitation to dinner". "Refuse" is stronger and implies a definite negative answer. Words such as "politely" or "graciously" are less appropriate with "refuse". Example: "The king refused to see the emissary of the country that had attacked his borders and instead sent an army to repel the invasion." A good way to understand "reject" is to know that it comes from Latin and literally means to "throw back". So, now, the actor is not just saying "No" but figuratively "throwing back" what what was offered. Example: "The prime minister rejected the peace proposal as offering far too little in the way of reparations." "The girl rejected the boy's marriage proposal." "The businessman rejected the proposed purchase of his company as being priced too low, considering other recent transactions." "Spurn" adds a sense of contempt to the actor's action. "Despite the fact that he was handsome and wealthy, the girl spurned the boy's advances because she thought he was arrogant and self-centered." "The prime minister spurned the peace proposal because he knew that the population of his country felt too angered and insulted to consider anything less than total unconditional surrender." "The businessman spurned the offer to purchase his company because he felt that it had not been made in good faith." Hope that helps.

There is some overlap between refuse, reject and decline, but this a general explanation: Reject is the opposite of accept. eg My visa application was rejected. Rejected is quite a strong word. It literally means to throw back, and it suggests that there is something wrong or inadequate about whatever is rejected. Compare with: eg The president declined the invitation to attend the dinner. This is much more polite, and would not cause offence. There is nothing wrong with the invitation (you wouldn't 'reject' an invitation). The president is simply not accepting it. Refuse simply means to say no. You could use refuse in either of the above situations. eg My visa application was refused. In this situation you don't know why they said 'no' to it. Maybe it was an acceptable application, but they simply had fulfilled their quota of visas for the moment. eg The president refused the invitation to attend the dinner. This would be seen as rude and insulting. 'Spurn' is a very rare word these days, and is not used in everyday language. It suggests a lady in a nineteenth-century novel who breaks a young man's heart by 'spurning his advances'. You definitely couldn't use it in either of the above examples!