@op you can have more than 4 letters if you want. i sent 4 to AADSAS. the extra ones i just sent separately
i dont think it really matters what kind of professor you get to write the letter? if your engineering professor taught a bio class or chem class, then i would consider them a "bio" or "chem" professor. if your engineering didn't teach any bio or chem class, then i wouldn't consider him a "bio" or "chem" professor.
i would just recommend choosing professors you are comfortable with. only you will know what type of relationship you have with the professor. i wouldnt really want to help you choose which professors to ask. i don't think it will make too big of a difference between the professors you choose.
as a generaly rule and this is in almost every thread. submit your application as early as possible and send in your dat scores later.