Old Zen on new PHP

I have 2 sites runing 1.3.7 on PHP4 hosting account. I've just installed a 3rd site using 1.3.9h but I'm having trouble because of the old PHP4 version. My host says he can migrate my whole account to a PHP5 server but all 3 site will have to go together since it's a multiple domain account. My question is if I do this to get the 2rd site running properly will the older 1.3.7 site then have problems because of the newer PHP version?

My eaelier 2 sites are quite extensively modified and I don't really want the hassle of upgrading those sites but I would like to use the latest version on the new site.....

My question is if I do this to get the 2rd site running properly will the older 1.3.7 site then have problems because of the newer PHP version?
Yes they will have issues
My eaelier 2 sites are quite extensively modified and I don't really want the hassle of upgrading those sites but I would like to use the latest version on the new site....
Your choice but you are waiting to be hacked
In the interim you might try this post #6
http://www.zen-cart.com/forum/showthread.php?t=153143

I've already tried the fixes in the post in your link for the new site on the old version. I solved most of the problems but completely lost the product edit page altogether.

The 1.3.7 sites have had all of the security fixes applied for the last couple of years and I'm comfortable that they are safe for now....

Looks like I'll have to get a seperate hosting account for the new site. that's a shame...