notsomuch

Hi everyone, I applied for 4 phd programs and one M.S. I got accepted in a phd in US with funding, and the deadline for their offer is April 15th. I did not get any result from any of the other programs. I like the university that I've been accepted in, and I don't want to lose the opportunity, but also there are one or two universities that I prefer, and if I get an offer from them later after I accepted this one, I will be really sad. I've been waiting for their answer for about 4 months! I don't know what's taking so long! So, what should I do? Should I email those universities and explain the situation? (if so, can you suggest a template for my email?) Or should I just be happy with this offer and accept it? Or should I accept this offer and later, if I get accepted to another university, reject it? What are the consequences of this action? + I can't accept an offer without funding, unfortunately.