Without a signal strength indication, it's pretty difficult to tell from speed or other secondary issues how well you're aimed.

Most all 802.11{b,g} AP's have a "Received Signal Strength Indication" variable (also known as "RSSI") kept in the firmware. Your task is to get this information.

Some AP's will present the RSSI to you as LED's on the front panel (very few do this -- cisco's Aironet does, last I looked), some other AP's will report the RSSI to their HTTP-based management pages, and some AP's will report the RSSI to SNMP (Simple Network Management Protocol) based network management programs in a device-specific MIB (Management Information Base).

It is often easier to find a host interface (eg, a 802.11{b,g} PCMCIA card) and software that reports signal strength. So what I'd recommend is to keep this simple: get someone to take a laptop with a host interface card and the signal strength indication on the screen to one end of the four-mile link, as close to the AP's antenna as possible. Aim the antenna from the other end of the link as needed. Switch people/equipment (have the laptop and person operating it come to your first location) and repeat.