Program uses 4Gb= $2^{32}$
Offset= 16 bit and for page number we can use 48 bits
Now pages number is split equally so we have 12|12|12|12
Page size is 64 KB so number of pages a program can have is $\frac{2^{32}}{2^{16}}=2^{16}$
Number of 4th level page table = $\frac{2^{16}}{2^{12}}=2^{4}$
1 single 3rd level page table
1 single 2nd level page table and
1 single 1st level page table
so total space = $2^{15}+2^{15}+2^{15}+2^{4}*2^{15}=19*2^{15}$
approximate value is 608KB
and if you are asking size of single page table then = 32 Kb
BONUS
if we use single level paging then total size= $2^{48}*2^{3}=2^{51}$ huge overhead
if we use 2 level paging of equal page no 24|24 then total size=$2^{27}+2^{27}=2^{28}$=256MB
3 level paging of 16|16|16 total size = $2^{19}+2^{19}+2^{19}=2^{21}$=2MB
hence 4 level paging is useful