What is the best tool (and my chances of success) for recovering files from an Ext4 partition?

chances are you will not be able to recover your data. In this situation it is likely that the previous data on the ext4 partition was overwritten. If your first partition size was originally 100GB then any subsequent imaging would need to be the same size otherwise it would cut into the 400 GBext4 partition. Any partition larger than the original 100GB will cut into the 400GB partition and result in the erasure of data. The 300 GB partition you created effectively wiped out the 400 GB partition because the two invariably must share some of the same sectors on the disk.

"Im sorry but I have to disagree with axxmasterr

You are able to recover data following a dd clone. I had a client who accidently performed a dd on x3 1 tb drives twice, (dont ask how he did this) and data recovery was 100% possible. We recovered all his files. The only issue we had was the size of the drives, being 1 terabyte called for a load of memory to run the data recovery analysis.

Use a tool called R-Studio. It runs from windows, mac or linux. It recovers any type of file system, more or less. You will be surprised what it recovers. It performs deep analysis of drives. I have never had any issue even after multiple formats on drives or mac, linux, bsd. Yes at some point the data will be re-written completely over. But until that point, its possible to get to close 100% back.

I dont work for r-studio to clarify."