I hope your team understands that you cannot "assure" quality by testing - quality must be designed and coded in, and "A" in QA stands for "Assistance" - you can provide information what is current status of quality, if it improves or not, and suggestions how to improve your internal procedures to improve quality in the most effective way.
Also, for a startup, time to market is the most important metrics, if your customers are willing to tolerate some glitches (but not too many, because you are building also brand name recognition. If users dislike what they see, they may decide it is not worth, and will badmouth your product even later when quality improved.)
It will take you time to establish process and procedures for automated testing, so you will likely start with manual testing, and try to automate parts which makes most sense: takes most time, are used in multiple areas/paths, most straightforward.
In my experience, you unlikely will be able to write automated tests to all the features 4 developers are able to implement - so most of your time will be spent manual testing. It may even make sense to have separate manual tester (who needs to understand problem area, but not how to write the code) and developer of automated tests (who needs to be a coder, but does not require knowledge of all the libraries which code app uses). Different skillsets.
Unit tests should be written by person who knows tested unit the best - it's developer, as he writes unit's code (or even before coding, test first, as in test-driven development - google is your friend). Developers should also help you establish continuous integration process - always do only what causes most pain and has best return to time invested. Don't worry too much about architecture (just avoid obvious stupid mistakes), focus on speed to market.
Quality is team effort. But you (as coder for testing) can specialize on automated UI tests (because they require specialized knowledge, unrelated to knowledge of internal core libraries of your system as your developers need) and tools needed to improve the process.