As I have trial versions of Sonic and H4, it looks like I can buy Sonic for $199, then upgrade to H4 for an additional $99. I wondering if I do this will I loose Sonic, i.e., will it get deactivated? Seems like a really good deal if I get both for $300.
Why you need Sonic? All sonic features and content is included in H4.
Take a look on http://www.steinberg.net/en/products/vst/halion/whats_new.html
no you will not loose sonic, you can even load sonic sounds in halion 4, i got the same deal, not bad. sometimes i will fire sonic if i just need a simple interface…
According the link I provided HALion 4 contain all from HALion Sonic => 
Waste of money for “Simple” UI in Sonic? HALion 4 is able to look like sonic - You can configure to your taste…
Halion is build upon SE, so you need the SE sounds for Halion, i just said sometimes you need a quick and fast instrument and its easier to call it under SE.
The original poster was asking about upgrading to SE then Halion, it works as the cheaper route then buying halion straight, so you will end up with SE and Halion, in face SE has 14 Gb and the Halion upgraded is about 877Mb over SE, GOT it!