Faouzia, I realise that technically an r11.3x Manager still supports the 4.5 Remote Agent. However I feel I must restate that this combination is not desirable, and should not be suggested as a possibility if it can be avoided at all.
For example, the 4.5 Remote Agent of course has to post data directly in to the AutoSys database. Under some circumstances the 4.5 Remote Agent does not release database connections properly. This can lead to all sorts of unwanted behaviour, such as connection pools becoming exhausted, or job statuses arriving in the wrong order (STARTING > SUCCESS > RUNNING). I have seen this stuff happen and it is not pretty.
Getting the new System Agent to sit alongside the old Agents is easy and is much the better approach.