20% of what? If the company is worth $100, then congrats go enjoy a few drinks this weekend. If this company is expected to be worth $100 billion dollars in the next 5 years, then you would be a very well compensated CTO.
The question of equity is how much you are bringing to the table compared to how much the rest of the company is 'worth' (in the case of a start-up, this is more of a future value rather than current cashflow)
The investors will get equity because they are infusing the business with cash. You get equity because it a) incentivizes you to make good business decisions and b) fills a compensation gap that you might otherwise make at a more established business. The founders get equity because this business was their idea.
They might be calling you a 'co-founder' but it sounds like they actually started it and got the business to this point without your help. As such, I think 20% is a big ask. They will probably balk at giving you so much right before they have to give away more to the investors. I could see getting 10-12% as a more reasonable goal. Although, this does depend heavily upon your location and the type and positioning of the business, so I could be way off.
You might also ask to be made an equal partner with the other co-founders. That is, you each get an equal share of equity (so if you are the fifth co-founder you get 20%). But that will also mean your equity will be diluted equally as investors and others come on board with the business. So you might end up back at 8-12%.
Update to address comments below
First, let me say that I am not a lawyer nor an expert.
If you accepted a 10% deal, I would expect you to either not be diluted at all, or maybe diluted a little at subsequent funding rounds and only in proportion to the equity you hold (i.e. if he holds 50%, you hold 10%, and another holds 40%, one dilution strategy to give equity to a fourth party that wanted 10% would be for you to give 1% (leaving you 9%), the founder to give 5% and the third to give 4%, but this all depends on the contract you sign and others have signed, so read that carefully)
I think a 4 year vesting is fairly common, but you could try pressing for 2 or 3 years instead or removing any cliff currently on the vesting schedule. The founder will still want some sort of vesting schedule to keep you invested in the company over several years (to make sure you don't just take the equity and then kick-up your feet and let him do all the work to make you money). Or you could save your bargaining for getting more equity rather than getting less equity more quickly.
In general I think you should sit down and ask how committed you want to be to this business. By your description, you could have a huge impact on the potential success of this venture. Also assess your risk tolerance during this. In a start-up, your job and your investments can be tied up in one place; and that place has a very real chance of total failure.
If you want to be very committed to making this a success and can handle the risk, then I would ask to be an equal co-founder with them (with equal dilution of your holdings when investors come on).
If this is more of a side project or you don't want the full risk. Stick to asking for 10% and do what you can and hope for the best.