The actual transaction has you described it (4 separate Gifts) has no Gift Tax implications. Period.
Because it was all done in cash and all deposited at once into one bank account, there is no proof that it was 4 separate gifts. So if the IRS were to question it (which is unlikely), they could asset it was not 4 separate Gifts. That is way creating some 'proof' (such as a Gift letter) may be a good idea.
For the future, if multiple gifts are given, avoid doing it in cash. Use checks or certified checks, with each person's name on the check, and a notation that it is a Gift.
