Hi Stanley
Not easy. It seems, if you believe the IGI, that none of Jean, Sara and the other John was baptized in Finchampstead. Although all were of this parish when they were married. It seems that they all lived, died and were buried in the parish.
It is sometimes possible to separate relationships from the age at burial and accompanying comments such as 'wife of' and 'widow of'. There are four possible entries in the Bershire Burial Index for your four Marshalls:-
John - 20 Nov 1806 no age given
John - 24 Jul 1831 age 82 - born about 1749
Sarah - 27 Apr 1804 no age given
Jane - 15 Mar 1829 age 80 - born ca 1749 - this fits Jean
It is not possible to be certain if we have the right people when no age is given. It seems that the other John may have been born in 1749.
Your best hope now is to look-up the burials in the parish register and hope that either the entry for John 1806 states widower, or the entry for Jane states wife or widow. {you can look at this parish register at any LDS Family History Centre}
An inscription on a grave stone could be equally informative.
It may be of interest that Edmund Marshall was buried 10 May 1777.
Best wishes
Colin