I do not think ...you will have a probelms with JDK1.4 ...
The only problems you will see with your app ex : webservices , java code ...
Ex : in your app code if you have any
word like enum , then this class won"t compile in 5 because this is a key word in jdk1.5..so you need to change the code ..in order to compile ...
i bet you will see lot of application problems and may need to update thirdparty jarsssssssssss
Deva
SCJP, WAS6.0, Sun webservices, IBM ILOG JRules 7.0 Certified