I know Sarria to Santiago might seem pathetic to those lucky souls who have heaps of time, but I absolutely loved that stretch of the route for my Camino. I went mid May, and was not concerned by the numbers of other pilgrims: there was good energy from everyone.
I didn't book accommodation in Santiago, and it was pretty full when I arrived, so I'd suggest pre-booking. The route was so pretty: farmland and hamlets and forest areas. A range of weather from really hot, to wet and chilly: not boring!
However 4 days isn't long, even for 112 km, because you are likely to be walking more slowly than you would at home (carrying a backpack, up and down inclines, rough underfoot e.g. stones and gravel, mud etc.) and you don't have the luxury of extra time for 'getting up to speed'....I walked very slowly (with time for wandering and reflecting and long café breaks) and chose to take 7 days from Sarria to Santiago.
I'd be inclined to decide according to your fitness and walking speed - and if you don't need a Compostela, you don't have to be bound by the '100 km' rule - you can start wherever you like. It's your Camino experience, after all. Thank goodness the medieval era is behind us: the Camino doesn't have to be penitential these days
Buen Camino.