Restore issue

It depends how your data has been written. If you've used multiplexing to do the backup (which is not good) then you'll not be able to read 4 streams simultaneously.

If you've not written the backups with multiplexing, and you've used 4 drives to store the data on 4 media then you should not face this issue.

Please confirm.

Do the streams that are queued say something like "waiting for media xxxxxx" " media in use"?

What does the status inside those jobs say?

How many rman channels was allocated when doing the backup ? my guess would be one

You get the best performance if you allocate the same number of rman channels for the restore as the backup was written with. 

To restore from 4 different media, you need 4 channels writing to each their own tape drive when doing the backup, which means you cannot use multiplexing on the tape drives.