Answers inline

1) Is classification analysis available in a Text Analyzer rule been renamed to Topic Detection?

Yes. It has now been renamed as Topic detection with 7.4. why the rename? Product feedback revealed that Sentiment, Topic(nouns) and Intent(verbs) Identification are all different types of text categorization. 

2) Also, the decision data rule under taxonomy of Topic Detection in the text analyzer rule that comes along with the product 7.4 doesn't seem to have a model in it. How do i update the decision data to have a model in it? Because text analyzer still ask if it has to perform model based analysis or Rule based analysis.

The expectation is to use the analytics center to update/create models. Also, the update on a checked out decision data will now link the update button to the analytics center link.

3) Is there a way were i can train the model that i use in text analyzer on the actual inbound data that i get from various channels?

Application>Channels & interfaces screen has the training data section to update data from email channel. 

4) When will i get records in Training Data tab of Email Interface?

Yep, the above step 3, mentioned the steps to list the training data from the emails. Once the CSR has identified the right topic, the data is then available on the training data tab.