When you come to Mumbai there are a few items you absolutely have to try - Pav Bhaaji, Frankie, Gola and a Grilled Sandwich. People will have their own opinions of where to go to have the best of these but since these are are relatively simple preparations you can't really go wrong.
Pav Bhaaji is made up of two main components - the pav (bread roll soaked in butter and some masala) and the bhaaji (potato and vegetable puree curry). It's a unique and simple meal but is full of flavour and is appropriate at any time of the day (as a snack or proper meal).
Frankie is a large snack in which a veg/non-veg curry is wrapped up in a paratha with a layer of egg. This concept is similar to a Mediterranean shawarma or the Kolkata Kathi Roll. In Lokhandwala I had the chili chicken frankie. It tasted amazing and had a relatively strong kick to it, making it a heavy snack. There are many different veg and non-veg frankies to choose from. This was my favourite in Mumbai.
Gola is basically a snow-cone on a stick. You are given a glass with syrup to dip the shaved ice into and then you suck the cold syrup out. You repeat this until the syrup is finished or the ice falls off the stick. The original syrup flavour is called "Kala Katta" (sour black) and is probably the most popular, but if you don't want to experiment then you can pick from a range of conventional fruity flavours. I had the chance to enjoy a Kala Katta gola as the sun set at Juhu Beach (one of Mumbai's tourist attractions) which made for some awesome pics.
Finally, the grilled sandwich. At midnight my friends and I went to Amar Juice Centre in Juhu which is known for its range of fresh juices as well as snacks. We ordered a grilled sandwich. You may be thinking "what is special about a grilled sandwich?". It is just like any other vegetarian grilled sandwich you get in NZ, the only difference being the sauce/chutney and cheese gives it a much stronger Indian taste. When you bite in to it you aren't expecting those tastes to come from a normal looking sandwich.
Apart from the popular meals listed above, if you have more time to spend you should go to Mohammad Ali Road in South Mumbai and try the mughalai food at a popular hotel such as Shalimar which serves delicious shami kebabs, tandoori chicken and bombay style biryani.
If you are travelling with a local who knows the area well, late at night get them to take you to a "Doodh wala" (milk seller). Apparently it is quite normal to go out late at night to buy some warm milk. At 1am my friend took us to this taxi stand where a Doodh wala (Arif) had his bike parked up behind some rickshaws. He gave me a cup of some hot bournvita milk. It actually felt quite relaxing to sip on a hot chocolate drink while we discussed the night's events. A truly different experience to say the least.
Four days in Mumbai was more than enough time to get a good feel of India's largest city and its cuisine. Mumbai is considered to be a generally safe and multi-cultural city in which many migrants come from all parts of the country to try living out the American dream. Once you are settled in Mumbai, you are no longer considered Kashmiri / Rajasthani / Punjabi etc - you become a "Mumbaikar" (similar to being a "Kiwi" in NZ). The "safe" and "multi-cultural" characteristics of the city are also reflected in the cuisine which creates a unique taste. Other than the heat, traffic/road conditions and the blind hatred Mumbaikars have for Delhi - it's a pretty cool city.