Remember that the amount you can remortgage to is governed by both your income and whatever loan to value you can achieve at the point of application in 48 months time.
eg: £100k value - remortgage 80% = £80k minus whatever you owe on the existing mortgage. They therefore want 85% of the equity being released by the process.
The only things that would make a major difference would be the increase in value over the 4 years..... or if you are expecting to be earning significantly more which would give you greater borrowing potential.
If you are not sure how much your property is likely to increase check
www.yourmortgage.co.uk they have a postcode level prediction. A typical remortgage L2V% is 80% during an IVA.
As with any mortgage you have to be able to afford the payments and this would govern your potential equity release.