I too had mine installed years ago and has the original FIT tariff (50+p per kWh). The smart export guarantee/SEG hardly worth doing as it pays 4-5p per kWh exporting to the grid, bear in mind a 4 kw system will only generate around 20-25 kw in a cloudless day at peak of summer, so unless you cover every single inch of your roof with panels you will never recoup anywhere close to what you paid out. You are better off installing an iboost to use the excess power to heat your water.
I recently had 3 x 6.5kw growatt battery installed for 10k. I only had it for about a month, but it's working really well, my past consumption was about 6kw per day with help from the panels, now it's around 1 kw daily. Obviously in the autumn/winter/spring when the panels aren't generating as much electricity the usage will increase, but I'm planning to sign up to an EV tariff after the summer so I can charge the batteries off peak to use during the day. I reckon I can recoup the cost in about 7-8 years.