You can do this in many ways here is an example 
 <form action="MainServlet" method="Post">
        SOME Name: <input type="text" name="someName" size="20">
        <br><br>
        <input type="submit" name="first" value="FirstServlet">
        <input type="submit" name="second" value="SecondServlet">
        <input type="submit" name="third" value="ThirdServlet">
        <input type="submit" name="fourth" value="FourthServlet">
    </form>
    with the following in MainServlet
    String sometext = request.getParameter("someName"); // your text box value
    if (request.getParameter("first") != null) {
        // Invoke FirstServlet's job here.
    } else if (request.getParameter("second") != null) {
        // Invoke SecondServlet's job here.
    } else if (request.getParameter("third") != null) {
        // Invoke ThirdServlet's job here.
    } else if (request.getParameter("fourth") != null) {
        // Invoke FourthServlet's job here.
    }