Load index to cache is not using memory available in the key_buffer_size variable
You have a pretty strange buffer configuration:
max_connections=4500 
. . . . .
key_buffer_size=62G 
read_buffer_size = 200M 
read_rnd_buffer_size = 100M 
sort_buffer_size = 1G 
join_buffer_size = ???  

While key_buffer_size is a global value for all MyISAM indexes across all the databases, (read|read_rnd|sort|join)_buffer_size are per-connection values. That amounts are reserved for each client attached to the mysqld service. The overall RAM consumption is calculated as:
(read_bs + read_rnd_bs + sort_bs + join_bs) * max_connection + key_buffer_size

In your case it seems to be at least 6TB in total that is too optimistic.
First you have to lower your buffer sizes to the reasonable values - 2-8MB each. Second you have to install mysqltuner.pl - a neat tool that analyze both config and status variables and propose a number of reasonable advices.
Third you have to perform a system monitoring for slow queries and on-disk temporary tables and tune your mysql accordingly to your certain needs.