The Board decides who to award the contract to.
The lowest price should not always be the one who is awarded the contract.
The Board needs to look at other projects that the companies have done, and talk to those references.
Some good questions for references:
How quickly did the company respond to issues after they were discovered?
Was a foreman on site each day and, if applicable, did they listen to your concerns and address them?
I would suggest contacting all four and set up a meeting with each contractor and the board to discuss the project.
Have questions handy based on their proposals.
Do not meet all of them on the same day unless there are several hours between meetings (this is simple courtesy.
Is sharing costs prior to awarding the contract unethical? In my opinion, yes unless you (the board made a decision and are bargaining because you prefer a different company). For example: When we bid out our trash service, we liked our current contractor. However, there was one bid that was 10K lower. We asked the current contractor if they would match, otherwise the contract would be awarded to a different company.
Is sharing costs illegal - probably not unless you stipulated the
bids would be sealed. In that case, an argument can be made that you made a contract when soliciting bids not to share.
Keep in mind that if companies find out that their bids were used to negotiate with another company for a lower price vs. actually considered for the contract, then word will get around and the Association may find fewer companies willing to bid in the future.