SQL Server – Does Database Backup Time Equal Restore Time?

Don't create the database on server B before restoring. Just restore it. It doesn't have to exist first in order to do that. This should work whether the database already exists or not (so perhaps you should consider using a better name than 'test'), just open a new query window:

USE master;
GO
RESTORE DATABASE test
  FROM DISK = 'E:\backup path\backup.bak'
  WITH REPLACE, RECOVERY,
    MOVE 'data_file_name' TO 'D:\data file path\test.mdf',
    MOVE 'log_file_name'  TO 'L:\log file path\test.ldf';

But I have no idea where you are getting a message about "busy" - are you restoring using the UI? Stop doing that - the UI is notoriously bad for assisting with tasks like backup and restore. And make sure you don't have any query windows etc. with their context set to the existing test database. To kick people out so you can restore over it, you can use:

USE master;
GO
ALTER DATABASE test SET SINGLE_USER WITH ROLLBACK IMMEDIATE;

To get the file names to place in the MOVE arguments, run this:

RESTORE FILELISTONLY FROM DISK = 'E:\backup path\backup.bak';

Since you said you are beginner and is in learning phase I would give you lot of good articles to learn about SQL Server backup.

To start with main question as already mentioned in my comment

So can I restore 4th one with 1st one. With out the 3rd ?

Differential backup only relates to previous full backup you have taken so NO you cannot restore 4th differential backup if 3rd full backup is lost. Also since as you said the database is in simple recovery model there is no chance of taking transaction log backup.

[To understand] Whether there any way if it is not simple ?

If your database were in full recovery you could have taken Transaction log backup. The good thing with transaction log backup is that Minimally, you must have created at least one full backup before you can create any log backups. After that, the transaction log can be backed up at any time unless the log is already being backed up.

So you could have restored first full backup and all log backups, in serial order, taken after the full backup. This would hold true even if 3rd full backup is lost.

You must read Understanding Backups in SQL Server there is a Backup myth buster series from PR related to backups you must also read that.

Feel free to ask if any doubts I would add the comments in this answer