LTO4 AND LTO5 tapes with same barcode

Yes, but as stated, ensure your barcode rules use the last 6 and not the first 6, otherwise you'll end up with a conflict.

you can do it by reading the last 6 caractes as noted but it is a bad idea since you will instead lose the ability to distinguish between tapes differing only in the first two positions such as  123456L5 and 223456L5. 

I have a Dell tape library that cannot append any suffix in the barcode, at the opposite the other HP library add the L3 / L4 to these tapes.

If you ever get a robot that only report the first six digits (e.g. Oracle) you have a problem. It's not consider "best practice" to have duplicate ID's in a barcode range - not even using the density type as part of the barcode.

Using the Density portion of the barcode only really would affect you if you had more than 9,999 tapes in any one density. Other than that as long as your library is able to differentiate between the first and last six digits used to find the tape it is up to you. Its really a matter of preference as long as those two conditions are meet. However if you are close to 10,000 media in a density I would recomend a different numbering system as in the long term it may cause problems.