A single 400-watt solar panel would make your off-grid solar system more compact. However, not very many single 400-watt solar panels are being manufactured yet. That is not to say that there are none. I found one that is manufactured by LG. However, I am yet to locate online sources for them. So if your search for the best 400-watt solar panel is for a single panel, then come back soon. Or reach out to LG.
Now that you are here, may I suggest you check out some single 300-watt solar panels and single 200-watt solar panels that you can combine to form your off-grid solar system.
This post will provide a list of 400-watt solar panel kits.