Difference between tra, wrth, gan & dan

As far as I am aware all these words mean 'while/as' but are used in different situations.

The only explaination I've seen suggests that gan/wrth is used for actions that happen at the same time as each other (simple enough), dan introduces an aspect (?) of the verb & tra introduces an independent aspect that happens alongside (?) the verb.

Unfortunately, I find this about as clear as mud. Every time I try to apply it practically it goes wrong.

Any tips appreciated.

Also - is the difference between wrth & gan merely dialectical?