Finding out if the 4th digit in a string is a number or character in r

Following on from the question as found by the link below.

How to test if the first three characters in a string are letters or numbers in r?

How do I include it to check that the 4th character is numeric also? For instance, an example of my dataframe is as follows.

ID   X
1   MJF34
2   GA249D
3   DEW235R
4   4SDFR3
5   DAS3
6   BHFS7

So again, I want the first three characters in the string to be letters and I also want the 4th to be any number between 0-9. If the given rule is achieved then I want it to paste the first three letters of the X variable in a new column. If not I want it to say "FR". Hence the final dataset it as follows.

ID    X       Y
1    MJF34   MJF 
2    GA249D  FR
3    DEW235R DEW
4    4SDFR3  FR
5    DAS3    DAS
6    BHFS7   FR

What I have so far that checks the first three letters is:

sub_string<-substr(df$X, 1, 3)

df$Y<-ifelse(grepl('[0-9]',sub_string), "FR", sub_string)

I have tried to expand it to read the 4th but doesn't seem to work.

sub_number<-substr(df$X, 4, 4)
df$Y<-ifelse(grepl('[0-9]',sub_string) && !grepl('[0-9]',sub_number), "FR", sub_string)

I'm probably doing something obviously wrong but can't seem to figure it out? Thanks in advance