ajax control toolkit versions

I am developing web applications using asp.net 4.0 / Visual Studio 2010.
I have included the AJAX Control Toolkit in my web project - version 3.5.40412.0
I see there is a newer version of the kit which is 4.1.50731.

1. is it ok for me to continue using the 3.5?  or is it best practice to use the latest?  
2. Each time I upgrade to the newer version of ajax, I would need to retest everything that uses the toolkit controls, right?  
3. I have an issue where I add the toolkit to the toolbox (as a new tab using choose items).  I can add it just fine.  But there are times where that tab simply isn't there anymore.  So I have to readd it.  

thank you for helping me!
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Miguel OzConnect With a Mentor Software EngineerCommented:
1. yes, unless there is special bug fix, feature that you need.
Best practice: I ususally wait until service pack  or minor version update. Reason: These versions are most stable that the first version targeting .net 4.0.
2. Yes, unless you have automated test, it is a lot of work.
3. I am not sure on this one. Is the toolkit tab dissapearing from the toolbox for the same solution or different solution? Can you see other tabs OK (e.g Standard,  HTML)?
ToolTimeGangAuthor Commented:
1.  Thank you.  good tip!
2.  Automated test...something to think about!  Good idea.
3.  Seems to be working now since I added it again.  Must have done something wrong the first time?  No worries.  I'm good now!
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