Let me think about this.
If all the letters and all the numbers were different then the number of permutations would be:
(This is assuming that number and letter positions are pre-allocated)
26*25*24*23*10*9*8*7=
$${\mathtt{26}}{\mathtt{\,\times\,}}{\mathtt{25}}{\mathtt{\,\times\,}}{\mathtt{24}}{\mathtt{\,\times\,}}{\mathtt{23}}{\mathtt{\,\times\,}}{\mathtt{10}}{\mathtt{\,\times\,}}{\mathtt{9}}{\mathtt{\,\times\,}}{\mathtt{8}}{\mathtt{\,\times\,}}{\mathtt{7}} = {\mathtt{1\,808\,352\,000}}$$
or
26C4*10C4*8P8
$${\left({\frac{{\mathtt{26}}{!}}{{\mathtt{4}}{!}{\mathtt{\,\times\,}}({\mathtt{26}}{\mathtt{\,-\,}}{\mathtt{4}}){!}}}\right)}{\mathtt{\,\times\,}}{\left({\frac{{\mathtt{10}}{!}}{{\mathtt{4}}{!}{\mathtt{\,\times\,}}({\mathtt{10}}{\mathtt{\,-\,}}{\mathtt{4}}){!}}}\right)}{\mathtt{\,\times\,}}{\mathtt{4}}{!}{\mathtt{\,\times\,}}{\mathtt{4}}{!} = {\mathtt{1\,808\,352\,000}}$$
Now, if they all have to be different but the number and letter positions are NOT pre-allocated then
$${\left({\frac{{\mathtt{26}}{!}}{{\mathtt{4}}{!}{\mathtt{\,\times\,}}({\mathtt{26}}{\mathtt{\,-\,}}{\mathtt{4}}){!}}}\right)}{\mathtt{\,\times\,}}{\left({\frac{{\mathtt{10}}{!}}{{\mathtt{4}}{!}{\mathtt{\,\times\,}}({\mathtt{10}}{\mathtt{\,-\,}}{\mathtt{4}}){!}}}\right)}{\mathtt{\,\times\,}}{\mathtt{8}}{!} = {\mathtt{126\,584\,640\,000}}$$
That is what I think anyway.