I've had by home brew sitting at 19c for 4 days now but there are no bubbles showing in the air Lock I opened the lid slightly to look inside I can see the yeast floating on top and there's a fairly strong smell of beer. Do I need to do anything or just leave it.
Take a gravity reading. Write down the gravity. Is it already in a reasonable range for a finished beer? Anything of 1.014 or lower should be fine.
Did you take a reading of the Original Gravity (OG)? Is the new gravity reading different? If yes, your yeast is working. If there is no change between the OG and the current gravity, your yeast may be dead. You should pitch another yeast.
The yeast may have died if you pitched it when the wort was too hot or if the yeast was uite too old or improperly stored.
You don't need to do anything.
Airlock activity is also a measure of how well your bucket/carboy/barrel/etc is pressure-sealing.
If there's yeast activity (foam, bubbles, etc.) and a beery-aroma, it's going well.
I would wait 2 weeks before doing anything. I expect you could then take a gravity reading and find the beer is finished. If you really want to, take a reading at 1 week. But be sure keep everything sanitary.
Also, if it's 4 days in and there was no yeast (not pitched, or no vitality), it would probably be already colonised by something nasty anyway.