I voted NO Only compatible with php5 is not a good idea. For someone who can change a host every now and then... its not a problem. He can search and find a provider for his needs. But for someone who has a dedicated server or a vps, with php4 on it and 100% of his accounts need and use only php4... it will be a big big problem.One more time: No to....... only php5 compatibility.
And i come up with another question. Is Elxis 2006.4 compatible with Php5 ?
Yes. I agree with you only on the question WHEN? But I think there should be no doubt on moving to PHP5. Some hosts even give you the option to choose wich version of PHP are you going to use. It is NOT difficult to upgrade PHP, as it is difficult to upgrade MySQL. No serious host will stay on PHP4 forever!