OK first off LTO4 drives cannot read LTO1 tapes, but the LTO3 can.
Next if the client also uses Retrospect then fine, if not you will need a copy of the program they use so the client can read the new LTO4 tapes.
From past experience I think that it is fine to just copy the data as is without modifcation.
The last thing is if Rettrospect or whatever application they use can do a tape to tape copy. If so you just copy straight from the LTO1 to LTO4 tapes. Otherwise it will be necessary to restore it all and then back it up again.
I think it is better to not do a one to one copy, instead take advantage of the increased capacity (1 LTO4 should fit 8 LTO1 tapes).
Use barcodes for the tapes and use that for a catalog of which LTO1 tapes are on which LTO4s.