IF Formula. Time between 2 dates taking into account Blank fields

Hello

I'm trying to work out how long a process is taking at several stages at the point the spreadsheet opens.

Formula I have is below.

=NETWORKDAYS(G4,S4,'Bank Holidays'!A1:A387)-1

I need to add an IF formula to show that IF S4 is blank then use today's date. This is in Cell B1. If it isn't blank then just use the date in the field.

Played around and I'm getting a bit stuck.

Help is appreciated.

Thanks
Matt

=NETWORKDAYS(G4,IF(S4="",TODAY(),S4),'Bank Holidays'!A1:A387)-1

Thanks I do need it to change everyday, mainly to prompt fields to be filled in.

I've now got a part 2 that has come about with this formula that is slightly separate. I need the calculation to look at 2 start dates.

IF Q4= "N/A" (Text) then use N4
IF Q4= "A date" then use Q4

Tried to start this off myself below, but need to add the counter

=NETWORKDAYS($M4,IF($Q4="N/A",$N4),'Bank Holidays'!$A$1:$A$387)-1

Help is always appreciated.

Thanks
Matt