I would like to ask more information about your problem if that is ok. Is the problem the same with all the 4 computers at the same time? I have a guess that those computers might be sharing the same IP address and also a DNS setup which might hinder your computer to open them. If it is a DNS problem, you could request for the DNS setup from the ISP.

Have you also tried using different browser to open the website? There are some websites that could be opened from a different browser, I'm not sure why though.