Hum, I feel it is a little small for farrowing. Piglets may get stepped on. You might want to consider a heat-box for the piglets so they can have their own space.
My mobile shelters are 6' wide by 8' deep.
I make them this size for multiple pigs, or farrowing where the sow can have one side and the piglets the other with a light. I could make the 6'x4' with no issues I'm sure, but I had the materials like you

and wanted options for it's possible uses.
Your shelter looks well built I don't think you'll have any problems in that regard.
All the best ~