Vaishali, the right branding agency for you depends entirely on where you are as a client. There is no single answer. In my experience, large agencies do not make the best partners for young companies. These agencies are set up to service large corporations and it is very hard for them to change their mind-sets and work culture, even if they want to. They will tend to give you safe, mainstream solutions even though that may not be what you need right now. You will get lots of ‘ideas’ on Power Point but I am not sure how robust these will be when it comes to execution in a startup environment. Of course, if you have very experienced marketeers on your team who have worked with big branding agencies before, then the engagement has a better chance of working out.
To answer your question about how to choose between the four, make your decision based on the specific people who will be in charge of your account. How many successes have those people had in your industry or with startups? Ask for details and references. Speak to their clients. Remember that projects that are showcased in their credentials could be done by any team around the world. The people who come to the pitch may not necessarily be the people who will create solutions for you. Make sure you know who’s doing the actual work.