Hi Kayssar,
You can upgrade like you indicated but there may be lot of changes that would have got introduced between these releases and hence the recommendation to go via an interim release, though you can go ahead with the mentioned plan as well.
The 11.4 version would not be available on the Juniper site, you will have to request for the same by opening a case with JTAC or your accounts team could help with the same.
Opening a case with JTAC would be easy and fast method to get the 11.4 version.