Lookup function with specific cells

Hello,

I want to have the last value entered for each column A B C D but I don't know how to do it.

It's not working when I select only the cells I want it to take into account for the formula for column A:

=LOOKUP(9^9;(L4;P4;T4))

but it just ends up with "#N/A" and I have no clue how to resolve this...
I've attached my sample file for tests.

I'd appreciate any help, thanks !