Does Bluetooth 4 or 802.11b/g/n penetrate through walls better?
Well, "I'm thinking of building a signal beacon designed to be used indoors."
Indoor can be an 3 bedroom apartment or an entire factory or a huge parking lot.

Majority of the Bluetooth Low Energy module manufacturers claim to have achieved a 100 m line-of-sight range with On-chip (No external) antenna.
I have evaluated 3 different BLE modules from one the most reputed wireless solution providers, and have managed to communicate to about 90 m line-of-sight and 75 m with walls and pillars in between.
All this while, the company WiFi was ON and  there were many a guys with cellular phones with their Bluetooth turned ON.

Bluetooth protocol divides the band into 79 channels (each 1 MHz wide) and changes channels up to 1600 times per second. Bluetooth implements Adaptive Frequency Hoping.
Read the story from Horse's mouth itself: Bluetooth standard page: Bluetooth Basics


  Interference: 
  Bluetooth technology's adaptive frequency hopping (AFH) capability was designed to reduce interference between wireless technologies sharing the 2.4 GHz spectrum. AFH works within the spectrum to take advantage of the available frequency. This is done by the technology detecting other devices in the spectrum and avoiding the frequencies they are using. This adaptive hopping among 79 frequencies at 1 MHz intervals gives a high degree of interference immunity and also allows for more efficient transmission within the spectrum. For users of Bluetooth technology this hopping provides greater performance even when other technologies are being used along with Bluetooth technology.
  
  
  
  Range:
  Range is application specific and although a minimum range is mandated by the Core Specification, there is not a limit and manufacturers can tune their implementation to support the use case they are enabling. Range may vary depending on class of radio used in an implementation:
   + Class 3 radios – have a range of up to 1 meter or 3 feet
   + Class 2 radios – most commonly found in mobile devices – have a range of 10 meters or 33 feet
   + Class 1 radios – used primarily in industrial use cases – have a range of 100 meters or 300 feet  


Adaptive Frequency Hopping (AFH) is a built-in coexistence feature that is found in most Bluetooth devices today. With AFH, a Bluetooth radio scans the operating band for interference and adapts its frequency hopping patterns to avoid DSSS channels. This decreases interference (and therefore increases performance) between Bluetooth and Wi-Fi radios.

Speaking of 802.11 solutions, the kind of modulation 802.11b inhibit is DSSS and that of 802.11g is DSSS/OFDM, so they can control their interference and susceptibility to interference by using signaling methods. 

So, to answer your real question "Does anyone know if Bluetooth 4 or 802.11b/g/n might be better for this application?"
I'd say, "It depends on what and how is the indoor term used. Be assured, interference is well managed and not an issue to fret about."