Hmmm, someone has breached a contract with you, that's for sure! Landlord contracted with all 4 of you to be jointly and serverally liable for the rent/deposit/damages. By unilaterally releasing 3/4 of the potentially liable people, he breached his covenant of good faith with you. This may have reduced his potential damages against you to 1/4 of the total.
Alternatively, you 4 all promised to each other to be jointly & serverally liable for the charges under the lease. This was an implied contract between you 4. Thus, while landlord can release them from lliability directly to him, YOU can still sue them for contribution and reimbursement for 3/4 of the total damages landlord charges you. Don't forget to include attorney fees and costs!
Since there's a lot of money involved, I suggest you hire an attorney to assist you.
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This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.