We have a deck off the back of our house – L-shaped, with the main part of the deck 14x32 foot and the ‘L’ 12x17. We want to put speakers out there. Nothing expensive as it’s mostly for background music during cookouts and such. Would it be better to have 4 smaller speakers (40 watt) or two larger ones (100 watts)?
I’m not sure there is a factual answer to your question.
Given the size of your deck, I’d venture an opinion that two speakers would be sufficient.
I just added speakers outside myself. You said in your OP that you didn’t want anything expensive, but the decent speakers I found were all expensive.
I ended up buying a pair of Klipsch outdoor speakers. (I first bought a pair of Polks, but the resistance for those speakers was too high so the volume compared to the indoor speakers was insufficient. And I love Polk speakers. They are what I have for my indoor home theater system.)
The Bose sounded OK to me too, but the ones I considered didn’t have a separate tweeter and woofer.
By the way, don’t bother with wireless speakers. I have some friends who’ve tried that route and have been severely disappointed.
I have a separate amplifier for my outdoor speakers. It’s only 15 W rms per channel. (Note to Crafter_Man: the “rms” terminology is useful, even if it’s wrong).
I can control the volume of the outdoor speakers independently of the indoor speakers.
And the volume on the outdoor speakers never, ever gets turned up beyond the “3” setting (i.e. where “0” is the minimum and “10” is the maximum. My amplifier doesn’t go up to “11” ;)).
Volumes louder than that make conversation impossible, and would probably really annoy the neighbours.
You could probably get by with 2 x 40 W speakers. 2 x 100 W would be needless expense, coz it’s unlikely that you’ll ever use them at full power.
4 x 40 W speakers could be useful, though, because it would allow a more even sound distribution. The volume coming from each individual speaker can be lower, while still distributing the sound throughout the deck area.
Teah I’ll second the Bose option. Plus if you have a good receiver in the house you can adjust the speakers accordingly. The option I used has the speakers under the eaves, to protect from the elements… It’s a good thing to think about…
O.K., you’re forgiven this time. 
The problem I see coming here is that you want inexpensive speakers but will probably want to use them at low volume to avoid those pesky police visits. Inexpensive speakers and low volume generally don’t go well together, in my experience. I would go with the aforementioned Klipsch, a pair of Cerwin Vegas, or a low-end pair of B&W. Bose used to have a problem with low dB input, but perhaps they’ve improved things.