It depends on where you are in your development phase. If most of the development work is done, and you have already done a lot of test work, and are close to releasing you application, you should stick with 3.4.6 for now.
If you still have a lot of development and test work to do, you should upgrade to 4.0 to take advantage of its extra features.
Version 4.0 is 100% compatible with 3.4. But like with all X.0.0 releases, you have the extra risk of some new bugs.
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Marco Kalter
Allround Automations