I have t-shirts and other garments made up all the time. Just put in an order yesterday in fact. You can probably create a design idea the best you can in something like word and send that to a screening place and they will design a screen
for you. Having a screen
designed and made for only 4 shirts will jack the price
up notably and as Paul said, you may do just as well, to buy some nice polo shirts or something and have them embroidered.
Have you considered hats? - The advantage is one size fits all, so you can just have a stock of them. You can also just have some on board for crew to wear and have a common look without even giving them away. I had some embroidered with a basic sailboat outline and the name of my boat under it.
After the basic set up, 2-color screened t-shirts cost me about $8 each. I just ordered some screened wicking fiber Ts for about $14 each.