Thanks for all the suggestions. Although they all sound very reasonable and wise, I have limited options for the wood. The 2x4s and 4x4 is all I have for this frame construction. Guess I'll go with my option 3 as it's the strongest among what I can do. I was hoping for other notched options to be okay, but what I'm seeing here, even option 3 will not be perfect. So, having limited resources, I'll go with a subpar design and will keep the fingers crossed.

Thank you. If you have a book name in mind I can look up online. Otherwise, will research myself. Thanks again for the suggestion. Nothing beats good old books for sure.

2x4s spanning for almost 10' felt a little sketchy. Thought it would help to put a stringer/beam (don't know the exact terminology) under them. The legs will be scattered around so that beam is more of a support rather than carrying all joists above.

Thank you for your suggestions. Any advice on this matter would be appreciated :)