Regnal dates

Some of old editions Burke's Peerage have "living temp Henry II, which is fairly obvious, but what does "living 5 Henry IV" mean, I wondered?

If it meant 5th year of his reign, but if so why not just say 1404?

I discovered that this is the system of regnal years which in medieval times was more commonly reckoned from 25 March (or sometimes from 25 December). In England, and later the United Kingdom, until 1963, each Act of Parliament was defined by its serial number within the regnal year in which it was enacted. In Canada, (continued)