the start.php and manifest.xml files have been replaced in favor of elgg-plugin.php and a Bootstrap class
see http://learn.elgg.org/en/stable/appendix/upgrade-notes/3.x-to-4.0.html#plugin-development
Then How would I load my custom CSS to the plugin without start.php? please help.
Just as before you can extend the core CSS view (elgg.css) with your custom css view http://learn.elgg.org/en/stable/guides/views.html#extending-views
To extend the core view you can use the elgg-plugin.php section 'view_extensions' http://learn.elgg.org/en/stable/guides/plugins.html#elgg-plugin-php
Most of the features you did in a start.php were of a static nature. Meaning they didn't change with plugin settings and/or the type of request. So doing most of this from a static configuration file save you a lot of work.
Think about registering plugin hooks or events or extending views etc.
Have a look at the docs about the elgg-plugin.php and see how much 'programming' you can save by a simple file ;)
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