I use a router that speaks IP bonding. My ISP specifically supports line bonding as a service and if one line goes down they redirect the traffic to the other lines and my router does a similar thing in relation to upstream. Since my lines’ upstream speeds are not equal, my router has been told by me how to split the traffic between the various lines; that is, what fraction to send to each according to individual speed values that I have given it. Regarding downstream, my ISP gets per-line speed values dynamically from BT and these change within a few seconds of the lines’ sync speeds changing and the sync speeds are down-converted by being multiplied by a suitable number, something link 0.87 iirc; I use 0.884434 * 0.95 * sync_rate (for ADSL2) as my chosen IP throughout rate (ie including the cost of IP headers and all other overhead).
To answer some questions you had from before: BT did a trial some years ago in the Isle of Lewis (Eilean Leòdhais) at Tolastadh fo Thuath where they put in an FTTC-type cabinet with long copper lines coming from it to the not that nearby houses. That contained LR-VDSL2 modems with a unique configuration, and it worked well, I believe, because (I assume) the tx power was high and there was no competition from old ADSL / ADSL2+ users, and I believe it worked well despite the fairly long distances from the houses to the LR-VDSL2 FTTC cab. However BT didn’t extend this trial into a widespread rollout because of the problem of existing users need ing to all agree to upgrade to LR-VDSL2 service (unless they just changed it without people’s agreement, as happened with ADSL Max), and certainly would need to upgrade their modems to handle LR-VDSL2 in some cases. I seem to remember that it was discussed in an old thread here somewhere, but memory fails me. Another reason why it wasn’t followed up my be that by then BT knew they were going to replace absolutely everything with FTTP at some point in the not incredibly distant future.
By the way, you don’t need four lines; bonding two together would be better than bonding four (even more efficient still), as in my experience the efficiency regarding upstream goes down a lot when you go from three lines to four - three lines or fewer has very good results. Having said that regarding downstream, the combined downstream results even for four lines are very good still, I feel. I wrote a thread about my combined efficiency some time back, with many many numbers in it.
I pay my ISP for IP-only (no voice) service on each line and then on top of that I pay a variable amount for download by total bytes downloaded but other charging schemes are available (I don’t understand all the options; it’s very confusing). Uploads are free (effectively), although with TCP an upload involves a certain small amount of return ACK (ie downstream) traffic. So the costs are a modest amount fixed per each line and a variable amount according to the amount you have downloaded each month. I do a lot of large downloads in the middle of the night (02:00-05:59 BST; ie 01:00-04:59 during BST summer time) because at that time downloading 1 TB [!] costs £3.90 (+VAT? who knows)
Since I have no voice service I have tried VoIP but it didn’t work too well because of the low upstream speed I think, or because of the dissimilar upstream speeds, so instead I have configured things using my ISP’s control panel (at clueless.aa.net.uk) so that VoIP coming into our phone number is redirected to my wife’s EE mobile phone. We have good 4G here, because despite the remote location (once you get out of the village, in which we are the northernmost house, onto the moor you can go 3.5 miles approx without seeing another house or wall or hedge or fence or even tree. And across the glen, in direct line of sight, we have the 4G basestation looking right at us.
One weird thing, discussed elsewhere, about my combined upstream is that it used to be (reported!) as a slightly higher 1.3 Mbps instead of the current 1.15-1.18 Mbps at times but not too long ago I was getting a reported combined result of 1.55 Mbps upstream. It could be or maybe should be due to changes in the speedtesters’ code.