transaction log backup on SQL server I am using log shipping as DR for my organisation. 

For the particular time like morning 4 to 5'o clock .wrk files are falling instead of .trn file and that particular file take too much time to copy. 

I configured log shipping for 570 databases in 4 instances in DR server. 

Please suggest.
.wrk files are just temporary extension names, while log files are being copied. If log is small enough you wont even notice that change, but in your case it is notable. 
Basically this is way of sql server ensuring that log that has being copied wont be accidentally restored by LSRestore job or by a manual restore.
As soon as log has been copied successfully it will change into .trn.

I dont know anything about your SQL, hardware and network specifications but having 570 transaction log backups in a span of one hour once in 24h could cause overhead on disk IO.
Check the log file size, perhaps adding to the schedule one more LS during 24h span would be beneficial (lower tran log size).