When connecting the DDJ-WeGO4/DDJ-WeGO3 with my iPad, can I use the iPad while charging?

Yes.

However, when using the DDJ-WeGO4/DDJ-WeGO3 on USB bus power, you cannot charge your iPad.
If you want to charge it, connect the DDJ with the AC adapter included in the DDJ-WeGO4/DDJ-WeGO3 accessories.