The default frequency for 4m built into the software is no longer correct. Currently the dial frequency required for the band is 70.0910mhz. This means the transmit frequency will actually be around 70.09250mhz. 

To change frequency simply type 70.0910mhz into the dial frequency window of the program, then click anywhere in the 'waterfall' display area and accept the change.

There is an ongoing debate as to whether the frequency 70.0910mhz should be the dial (i.e. receive) or the transmit frequency. It appears that there are errors in the bandplans published by different organisations that deal with such things. It is, therefore, likely that this frequency will change again in the near future.

As of today 70.0910mhz is the frequency the few stations on 4m are using. (July 2012)

However a quick check of the 4m map will show whether this is true in the future.  It is important to check first. On the other bands we just take the software settings for granted. As with many things on 4m Wspr, frequency cannot be taken for granted.

Radio Off Frequency?

Many mutlimode multiband radios obtain frequency settings from a central oscillator (simplified explanation but useful) which is then multiplied up for different bands.  Any error in this oscillator will become more apparent with each multiplication. This is especially critical at 50mhz and more so at 70mhz. Anyone who has watched the 50mhz Wspr window for any length of time during an opening will notice 'worms' right outside the window. This may indicate that operators are unaware their radios are not actually on the correct frequency, even though the dial is set correctly.

A few hours observing and it is possible to deduce the inaccuracy of your radio (if any). The best way to correct is to adjust the receive dial a few hz at a time until your spots agree with the spots of others. GM4FVM also pointed out that there is an average frequency for each station in the 'Activity' window of the Wspr website. This can help if you adjust your radio dial for a best fit.

Once you have an idea of the error you have various choices:

1) Leave your radio dial there. Easy and it works. I ran my FT847 for many months with the dial set 100hz low, this put it more or less in the centre of the window. However it does prevent frequency hopping.

2) Recalibrate your oscillator. Having access to a Flex 1500 with PowerSDR panadapter screen, and a good signal from GB3BUX, I finally managed to recalibrate the oscillator in my FT847.  It was simple, but an accurate source is required. Details of this adjustment can be found online here under
'Tweaking The Reference Oscillator' for the FT847.

3) Other correction method of your devising...