Reverse osmosis works on pressure differences, so you're going to lose 4m of pressure difference (roughly 4/10 atmosphere, or just under 6psi). That means that you might end up with higher TDS. You should be able to set the back pressure on the waste to keep the recovery rate in whatever range you want.
If it's going into a tank, don't have the feed pipe submerged in the pure, or some will almost certainly syphon back and contaminate your RO. I had contamination problems in the past and at one point had to replace a membrane after three months. Avoid.
One solution might be to have a small subtank by the RO and have a small pump lifting that water the 4m. A shurflo would do the job at my pure production rates (about 3lpm). As long as it's self priming it won't matter if the subtank empties so you can run it fast enough to ensure no flood.
Vin