Commercial transactions should make the incentives of the parties clear. Here, you are giving away something (20% equity) for a promise, which is bad for you. If they don’t deliver on their promise of bringing clients, and they still vest, that sucks for you.
Both you and the other party, may be better to have a commission based structure. You would be better off because you only pay if they deliver on the promise. They would be better off because minority shareholders can always be screwed, also if you “reinvest” profits then they get no dividends. If they refer you clients, then you could contractually give them a percentage of raw profits after expenses. You might allow them to “invest” those profits for equity? That keeps you both honest, and both incentivised to grow the relationship.
I suggest you spend some time thinking about what reasons a business would give away a future income stream and a percentage of control. Many people have a very naive view of what equity is. If some act can change annual profits from $1x to $2x then you might consider giving away up to 50% of equity (percentage depends on risk/volatility/probability-distribution, ignoring value of control, ignoring opportunity cost, other huge factors too).
Also think about how you split equity if you add 5 equity holders, where each holder can double profits. Hint: https://news.ycombinator.com/item?id=31733393 However, beware, because the party with all the power can usually snarf all the profits (typical employer employee relationship) and/or remove all their risks (e.g. get a percentage of contract price, rather than percentage of profits).
You also mention: “Currently the company has 4 software engineers as parters with each owning 25%.”. This is rather unstable because if one person reneges or underdelivers, you have a fucking serious governance problem. Vesting is the usual solution, but even that has some really perverse incentives. I have wondered about having monthly pay include X shares, that way those that work and then quit will get equity, but then get diluted over time. The X shares would need to decrease over time (probably non-linearly) since risk is high at start and lower over time. Maybe equity is some metric of $ invested (opportunity cost per person for the income they could otherwise earn) against risk.
As a generalisation, I suggest looking at the equity or pay structures of other consulting professions (legal, accountancy partners, engineering). Startup equity structures might not be a good fit for a software consultancy.
Another heuristic: if any HN user’s comments particularly resonate with you, search for the user’s past comments on https://hn.algolia.com/ and use a keyword to narrow down results. There is a lot of value in the back-issues of HN!
Disclaimer: Definitely my area of expertise, most of the above is ivory tower thinking and only a little is from practical experience. But I was involved with equity with a similar split to what you mention, so I have had more incentive to think about it deeply, and especially the downsides in retrospect.