4 month sleep regression - possible a very stupid question!!!

[maldivemoment]

...but do you count the 4 months as around 16 weeks or do you go with dates, as in 4 months since the date baby was born?

(drops head in shame in case this is silliest question ever posted on Mumsnet!! )

4 months is 17 weeks I think?

IME babies are either entering a sleep regression or in the middle of one for at least 12 months so a week here or there won't matter