To answer the question it is important to disentangle the proof as follows.

**Theorem 1.** Every minimum counterexample to the 4CT is an internally 6-connected triangulation.

**Theorem 2.** If $T$ is a minimum counterexample to the 4CT, then no *good configuration* appears in $T$.

**Theorem 3.** For every internally 6-connected triangulation $T$, some good configuration appears in $T$.

See the actual paper for the definitions of these terms. Theorem 1 does not require computer assistance, while Theorem 2 and Theorem 3 both do require computer assistance. According to this version of the paper, Theorem 3 can in principle be checked by hand. Indeed it is explicitly mentioned that

It can be checked by hand *in a few months*, or a few minutes by computer (this was about 15 years ago though).

I quote more on Theorem 3:

For each of these five cases we have a proof. Unfortunately, they are very long (altogether about 13000 lines, and a large proportion of the lines take some thought to verify), and so cannot be included in a journal article.

Theorem 2, on the other hand *really* requires a computer. From the same paper,

The proof of Theorem 2 takes about 3 hours on a Sun Sparc 20 workstation and the proof of Theorem 3 takes about 20 minutes.

Thus, given that it took a computer 9 times longer to verify Theorem 2 than Theorem 3, and Theorem 3 apparently can be verified by hand in a few months (let us define few=3), then under some very dubious assumptions we have the ballpark answer of

**Ballpark Answer.** 30 months.