Whoa Guys!
I take it that its a commercial site that you're trying to book?
if it is the owner has every right to impose whatever terms & conditions he may want to, we may not like it but there it is, don't forget that he has to make a profit to stay in business & its his business decision to impose a "mininmum number of nights stay" during the most popular periods.
Its not uncommon, either here in the UK or on the continent to impose minimum nights stay conditions during the "high" season periods, its the owners way of ensuring high occupancy during these times.
Now having said that, its been my policy to personally contact such sites & ask to speak to the owner and explain why I'd love to stay at their site but due to "work" commitments can't stay for the minimum night requirements say in your case 4, but I can stay for 3 guaranteed! & would they "waive" the minimum number of nights charge, 9 times out of 10 its worked! & the odd ones where its not I've politely explained that I'm sorry that they take that stance, but in future they will not get my business. their loss, not mine.
try it, you never know, you could be lucky!