You can access the source code via GitHub if you follow these steps: https://www.unrealengine.com/en-US/ue4-on-github

Did a quick `cloc` run of the codebase, shows the following:

    github.com/AlDanial/cloc v 1.84  T=86.60 s (949.0 files/s, 297044.5 lines/s)
    ---------------------------------------------------------------------------------------
    Language                             files          blank        comment           code
    ---------------------------------------------------------------------------------------
    C++                                  20720        1622066        1102650        7943910
    C/C++ Header                         46048        1443245        2403181        5914335
    C                                     3123         311091         384178        1319332
    Python                                2625         164396         219571         693345
    C#                                    2587          71616          99526         472242
    XML                                   1429           4589           4985         420621
    HTML                                   518           9623           1370         124409
    MSBuild script                         225             10            551         122808
    JSON                                   231             59              0         122326
    CMake                                 1655          18686          38278          97625
    Bourne Shell                           348          11335          15567          69747
    Objective C                            351          12507          15181          59608
    Objective C++                          124           7159           3992          34551
    Expect                                  20           3889             46          32132
    INI                                    341           5658           1719          27645
    make                                   682           9220          13527          25996
    Markdown                               156           7829              0          25333
    Windows Module Definition              145           1871            144          23754
    CSS                                     32           2699            553          23212
    Java                                   102           3035           3832          18332
    Pascal                                  15            798          26566          15507
    JavaScript                             103           2125           2208          15486
    Perl                                    76           2253           2218          11634
    HLSL                                   123           2518           3934          10942
    DOS Batch                              238           2492           1080           7361
    TypeScript                              32           1550            983           6242
    Windows Resource File                   70           1013           1348           3628
    GLSL                                    38            685            487           2994
    XAML                                    13            201             87           2861
    ASP.NET                                  7            538              7           2687
    Ant                                      4             54            216            799
    SAS                                      1             14             22             32
    Bourne Again Shell                       1              0              0              9
    ---------------------------------------------------------------------------------------
    SUM:                                 82183        3724824        4348007       17651445
    ---------------------------------------------------------------------------------------

Is that for UE4 or UE5? In any case, thanks for the reply.

Not sure, the git remote is `git@github.com:EpicGames/UnrealEngine.git` and the commit at the time I run cloc was `f8f4b403eb682ffc055613c7caf9d2ba5df7f319` which is the head of the `release` branch.

But, if UE4 was/is mainly, you can expect UE5 to follow the same path, they are not gonna rewrite 20720 files into a different language just for fun.