Dual pump system with a counter that changes between duty pump and standby every few times it runs
A micro-PLC (programmable logic controller) is ideal for this. They are very flexible, come in a variety of voltages (24 V DC, 110/230 V AC), relay or transistor outputs, etc., are industrially hardened and user programmable. Many allow writing of the program from a keypad / LCD on the unit or via PC software.



Figure 1. A typical DIN-rail mount micro-PLC.

Your program can be written in Ladder Logic - a diagramatic representation of an electrical circuit. In your case it might look like this.

' Turn on duty pump when FloatMid turns on. 
' Hold on until FloatLow turns off.
   FloatMid                                     DutyPump
----| |-------------------+-----------------------( )----
                          |
   DutyPump     FloatLow  |
----| |-----------| |-----+

' Turn on standby pump when FloatHi turns on. 
' Hold on until FloatLow turns off.
   FloatHi                                      StandbyPump
----| |-------------------------------------------( )----
                          |
   StandbyPump  FloatLow  |
--- | |-----------| |-----+

' Counter 0 counts first four cycles. Couter 1 the next four.
   DutyPump                                        K4
----| |------+-----------------------------------[CNT 0]---
             |
             |   CNT 0                             K4
             +----| |----------------------------[CNT 1]---

' After eight cycles reset both counters to zero.
   CNT 1                                         CNT 0
----| |--------------------------------------+---[RST}-----
                                             |
                                             |   CNT 1
                                             +---[RST]-----

' Pump 1 should run for the first four cycles or in standby
' for the next four.
   DutyPump      CNT 0                           PUMP 1
----| |-----------|/|-----+----------------------( )-------
                          |
   StandbyPump    CNT0    |
--- | |-----------| |-----+

' Pump 2 should run for the second four cycles or in standby
' for the first four.
   DutyPump       CNT 0                          PUMP 2
----| |-----------| |-----+----------------------( )-------
                          |
   StandbyPump    CNT0    |
--- | |-----------|/|-----+

' Give an alarm if FloatHi turns on. Latch until ACKN button
' is pressed.
   FloatHi     ACKN                              Fault
----| |----+----|/|------------------------------( )-------
           |
   Fault   |
----| |----+


Notes:

--| |--          Contact. Power flow when TRUE.
--|/|--          Contact. Power flow when FALSE.
--( )--          Coil. Can be an internal "flag" or a physical output.
--[RST]--        Reset coil.
--[CNT x]--      Counter. Starts at zero and increments on each positive-
                 -going edge of the input up to the preset value. When the
                 preset value is reached the counter turns on.


Examine the options carefully before you buy and try the demo software. Once you get started on this you will find many more applications so you should choose a brand and device that will meet your future needs.